|The Development of the German Verb Classes from Germanic to Middle High German|
German vocabulary in general, and the verbs in particular, have undergone several different types of vowel alternations which have changed the look and sound of the language. Three in particular -- mutations, apophony (Ablaut), and gradation (Abtönung) -- have been very important in terms of verb classes.
Mutations, at least as regards the German verbal system, are defined by Keller as "modifications of a stressed vowel under the influence of another vowel in a subsequent syllable."  This change is rather straightforward and leads to the differences in such verbs as stechen -- sticht, graben -- gräbt, and even voll -- füllen. The Ablaut, however, is a much more ancient process, more peculiar to German, and in many ways more important than mutation. Ablaut, which can also be called apophony, is "the regular alternation of certain Indo-European vowels in etymologically related morphemes."  In this phenomenon, vowels can occur in several different fashions, or grades: Normal Grade, in which nothing besides the normal German vowel shifts have changed the Indo-European vowel; Reduced Grade, in which the vowel is shortened when no stress falls on it (this can also result in Zero Grade, in which the vowel completely disappears); and finally Lengthened Grade, in which the vowel can be lengthened and can even absorb other unstressed elements. Thus, for the Indo-European vowel e, the following possibilities exist for it to appear in in German:
Abtönung, or "qualitative gradation," is a result of the changes that occurred in Indo-European as this language supposedly developed, and then proceeded to lose, a pitch accent. In this phenomenon, when a vowel had a secondary pitch accent, in IE, it would change from, for example, e,e to o,o in Gmc. This was a relatively narrow change, since the only vowels that were truly affected by it were e and a, but with the combination of mutation, Ablaut, and Abtönung, it is obvious that the alternations could become extremely complex, as indeed they did. A combination of the effects of these changes, for example, is responsible for the differences in such forms as the following: 
Several aspects of verbal morphology have remained the same from the Indo-European base up through modern German. In Indo-European, the verb was inflected to show person and number (though not gender), and aspect or tense as well as mood. Several phonological morpheme markers have been retained in German from Indo-European, as Keller states: "The clearest reflex of Indo-European is to be found in the personal endings of the present indicative: -s- of the second person singular, the dental of the third person singular and of the second person plural, and the nasal of the first and third persons plural."  Also inherent to Indo-European was a system of grouping verbs by thematic classes, according to the vowel grades which would be added to the root for different conjugated forms. This was one aspect that was to become extremely important in later Germanic groupings, for, as Keller states:
In Germanic, then, there developed a system of verb classes, descended from its Indo-European beginnings, but much modified. Eventually, three different classes of verbs emerged. The first class, the so-called weak verbs, or as Keller prefers to call them, the dental-suffix verbs, were an innovation of Germanic, and are therefore the newest development in terms of German verbs. These are the least defined verbs semantically, and they include a wide range of variation in both meaning and sound; this is also the most open group, with most new or innovated verbs after Germanic falling into this category. The second type of verbs, the strong or apophonic type, are semantically more determined and form a more coherent group; it is these verbs that fall into the familiar seven classes which we know today in German grammar. These verbs are in general more basic to everyday communication and are restricted, meaning that few new verbs in the language will be taken into this group. The third and final type of verbs is a mixture of the two previous groups; Keller chooses to call this group the apophonic dental-suffix type, although the more common designation, based on grammatical reasoning, is preterite-present verbs. Verbs from this category are the most basic in the language, and this is the most archaic type of verb that still exists. This group is completely closed, which means that no new verbs will be added to it, although verbs can be, and have been, lost from its ranks.
The class of Germanic verbs known as weak verbs (a designation given by Jakob Grimm) is the most linguistically diffuse group of verbs, although grammatically the easiest to explain. It was, as previously stated, an innovation in Germanic, and proved very important and persistent. The principal verbs from Indo-European which were absorbed into this new class in Germanic were the causative verbs with stems of the *-éje / éjo- form, as well as the inchoatives (verbs expressing the beginning of an action or the process of becoming). Within this type of verb there arose almost immediately several different classes, based on morpho-phonological rules, as follows: 
The second type of verb in Germanic was the strong or apophonic verb. According to Keller, the strong verb type "consists of a core of about two hundred and thirty verbs which are found more or less in all ancient Germanic languages plus perhaps another hundred which are only fragmentally or dialectically attested."  This verb class has as its hallmark feature the vowel alternation in different forms of the conjugated verbs, making a distinction between tenses as well as person and number. This gradation became then the standard for classifying the different Germanic verbs into the seven classes by which we know them today. Thus, for comparison purposes, it is helpful to examine the following table: 
The third type of verb to be discussed, the so-called preterite-present verbs, is a very small group, yet it includes some of the most basic verbs of the Germanic languages, most notably the modal auxiliaries. This group has in most cases an origin based on the Indo-European perfect tenses which, in Germanic, came to function as present-tense verbs. These verbs have several interesting features, which Keller concisely set out as follows: 
The modal auxiliary kunnan (NHG können) is an example of this third class of verb. Its conjugation, in Gothic, is as follows:
Old High German
As is apparent from the tables above, the largest change in the structure and shape of the German verb occurred in Old High German. There were inflectional ending changes, many of them due to the phonological changes that affected the language as a whole. Also, there were changes in the tense system of the OHG verbs, such as the development of the periphrastic forms for the passive and perfect tenses. In addition, it must be noted that the use of the ga-/ge-/gi- prefix on past participles became much more widespread. This development occurred as follows:
The past participle prefix ge arises from the use, in the first instance, of ge as a prefix to certain imperfective verbs to denote the idea of completion (a few such pairs are still seen in NHG though not with the old significance: bieten, gebieten, brauchen, gebrauchen). The ge, then, occurring so often in the past indefinite tense came to be associated with the ides of completion denoted in the past participle. In OHG still, perfective verbs like bringan, findan, quëman, wërdan, etc., did not have the prefix (gi in OHG) in the past participle. 
In terms of the verb classes, the subdivision of the weak verbs in OHG continued to be into four separate classes, although these varied slightly from their Germanic predecessors. Paul and Mitzka describe these classes as such:
In the first class of these verbs there occurred quite frequently a vowel mutation that differentiated the present tense from the preterite, for example sezzen -- sazta. As Paul and Mitzka explain, this alternation has come to be called Rückumlaut:
What arose, then, in OHG, was a categorization of weak verbs into the following classes: 
Although these classifications were fairly concrete, it should be noted that, as always, there was widespread dialectical variation, and, in addition, several movements between the different classes occurred over time. Keller claims that although the classes were not designed to semantically differentiate the verbs, nonetheless "in the classes Ia and Ib there were many causatives, e.g. tiuren 'to make dear', sougen 'to cause to suck', and in class III there were many inchoatives, e.g. fulen 'to become rotten', bleichen 'to grow pale'." 
The classification of strong verbs did not change all that much between Germanic and OHG, although it is interesting to note that there were some interchanges between classes. Some verbs from Class V (e--i--a--a--e) had joined class IV (e--i--a--a--o); the most notable example of this is brechan--gebrochan. Class VII underwent the most changes in regards to phonological developments; instead of the varied possibilities that had existed in Germanic, this class simplified to have the diphthongs ia or io in all of the preterite tense forms. As in OHG, this class, known as the Reduplicating Class (stemming from its original members), had a relatively small number of verbs in each of its sub-classes, but there was great variation in the vowels in the present stem, e.g. ei, ou, a, a, uo, and o were all present-stem vowels in these verbs.
In about two dozen of the strong verbs, according to Keller, phonological changes occurred, mostly of the forms d : t, s : r, and h : g ; so, for example, snidan--gisnitan, friosan--gifroran, and ziohan--gizogen. Interestingly, one verb was added to the strong verb class around this time: scriban 'to write' was added to Class Ia by analogy with the other verbs; it stems actually from Latin scribere. Loss was, naturally enough for a closed system, more common. In fact, by the Carolingian period about one-fourth of the Germanic strong verbs had been lost. Some of the losses are surprising because of their basic meaning and centrality to any language, but, it can be explained, these verbs were not really lost, but replaced by other, often weak, counterparts. Thus, some of the more important losses were: 
The so called preterite-present or apophonic dental-suffix verbs had been reduced slightly by the OHG period. They now numbered nine, and, as in the past, their present tenses clearly showed evidence of borrowing from the preterite forms. It is also interesting to note that the preterite forms, which are formed by the addition of a dental suffix to the 'reduced grade stem' (the present tense vowel in reduced grade), have no medial vowel between the stem and the -t. This stands in sharp contrast to the other verbal morphology at the time, although now, with the phonological occurrences since, it is not quite as striking. Thus, the preterite-presents, consisting almost entirely of modal auxiliaries, were as follows:
Not surprisingly, due to their basic meanings and unique place in the language, these verbs have withstood fairly well the ravages of time. As Keller states:
Among the preterite-present verbs only those particularly restricted in forms and use, e.g. eigun 'they have', genah 'it is enough', gitar 'I dare' have vanished and the OHG impersonal toug 'it is useful' has become an ordinary weak verb (taugen) as has an (and gi-an) -> gönnen. The others have survived and constitute the important group of the modal auxiliaries. Semantically, of course, they have changed much. 
Middle High German
The developments in Verb Classes from OHG to MHG are not nearly so interesting the ones before, yet they are still worth noting. They are, happily, fairly well-understood and easily explained. As Keller says, "nearly all the changes form the OHG to the MHG verbal system can be put down to the vowel neutralization in inflectional endings and, to a lesser degree, to apocope." 
In the weak verbs, the class divisions were greatly changed by the vowel changes mentioned above. Whereas Gmc and OHG weak verbs had consisted of four distinct classes (Ia and Ib being counted as two separate classes, due to the difference in mutation and endings), MHG had only three classes. The first, characterized by the suffix -te in the preterite and involving vowel alternation (mutation), was, in Keller's words, "a large class and particularly characteristic of MHG."  It included such verbs as brennen--brante and denken--dahte. The Rückumlaut of these verbs in OHG was for the most part discontinued, although it survived in certain verbs such as zeln 'to count' and retten 'to save'. Also, consonant changes between present and preterite usually were leveled out, as was the case with decken 'to cover', OHG preterite dahte, MHG dakte.
The second weak verb class consisted of verbs ending in the preterite suffix -ete, but which had no vowel changes in the different tenses. This class fused the OHG classes Ib, II and III, and was quite regular, containing verbs like baden--badete and lernen--lernete. Keller reports that in "conservative areas," the suffix could still be found as -ote, a holdover from OHG.
The third and final class of MHG weak verbs had the preterite suffix -te, just as Class I, but had no vowel alternation; thus, for example, verbs such as schamen--schamte and teilen--teilte. These verbs were remnants of the OHG classes Ib, II, and III in which syncope had occurred, so that the suffix became -(e)te. The levelled out nature of this class made it likely to attract verbs from the other two classes, since in class III no vowel alternation was necessary. Indeed, several class II verbs became class III during the MHG period, for example leben 'to live', lebete--lebte.
The strong verb classes seem to have undergone no major changes in the period from OHG to MHG, except for a "coalescence in class VII in consequence of a phonological change."  There was regional variation in regard to phonology and grammar, but in general the strong verbs remained quite regular.
The preterite-present verbs also stayed fairly similar to their OHG counterparts, but there were some instances of levelling and even replacement. All the modals now had infinitives, taken mostly from the preterite tense forms, although mutated or umlauted in the case of künnen, dürfen, mügen, and müezen. OHG unnan was replaced by the related günnen, and as Keller states, "structurally, though not historically, wellen 'to want' belongs here as well."  Thus, the MHG modals and other preterite-presents were: 
Thus, the development of the German verb class system has been complex and varied, but a truly interesting phenomenon.
Written and © Nancy Thuleen in 1991 for Linguistics 130 at Pomona College.
If needed, cite using something like the following:
Thuleen, Nancy. "The Development of the German Verb Classes from Germanic to Middle High German." Website Article. 31 October 1991. <http://www.nthuleen.com/papers/130paper.html>.