Germanic verbs

The Germanic language family is one of the language groups that resulted from the breakup of Proto-Indo-European. It in turn divided into North, West and East Germanic groups, and ultimately produced a large group of mediaeval and modern languages, most importantly: Danish, Norwegian, and Swedish ; English, Dutch and German ; and Gothic.
The Germanic verb system lends itself to both descriptive and historical comparative analysis. This overview article is intended to lead into a series of specialist articles discussing historical aspects of these verbs, showing how they developed out of PIE, and how they came to have their present diversity.

Verb types

The Germanic verb system carried two innovations over the previous Proto-Indo-European verb system:

Simplification to two tenses: present and past.
Development of a new way of indicating the preterite and past participle, using a dental suffix.

Later Germanic languages developed further tenses periphrastically, that is, using auxiliary verbs, but the constituent parts of even the most elaborate periphrastic constructions are still only in either present or preterite tenses.
Germanic verbs fall into two broad types, strong and weak. Elements of both are present in the preterite-present verbs. Despite various irregularities, most verbs fall into one of these categories. Suppletive verbs are completely irregular, being composed of parts of more than one Indo-European verb. There is one verb that is in a category of its own, based on an Indo-European "athematic" form, and having a "weak" preterite but a "strong" passive participle.

Strong verbs

Strong verbs display vowel gradation or ablaut, that is, the past tense is marked by a change in the vowel in the stem syllable. Examples include:
Modern English:

fall – fell – fallen
sing – sang – sung

Old English:

fallan – feoll – feollon – fallen
hātan – hēt – hēton – hāten

Old High German:

fallan – fiall – fiallun – fallan
heizan – hiaz – hiazun – heizan

In the older languages, these verbs may also be reduplicating, that is, the past tense forms add a prefix with the same consonant as the first consonant of the stem syllable. An example in Gothic is lētan – laílōt – laílōtum – lētans.
In Proto-Germanic, consonant alternations known as grammatischer Wechsel developed, as a result of Verner's law. This involves an originally regular change in the consonant at the end of the stem syllable. An example in modern Dutch is verliezen – verloor – verloren
These are the direct descendants of the verb in Proto-Indo-European, and are paralleled in other Indo-European languages. Examples include:

leipō – leloipa – elipon
fallō/fallere – fefellī – falsum

All Indo-European verbs that passed into Germanic as functioning verbs were strong, apart from the small group of irregular verbs discussed below. The preterite of strong verbs are the reflex of the Indo-European perfect. Because the perfect in late Indo-European was no longer simply stative, but began to be used especially of stative actions whose source was a completed action in the past, this anterior aspect of it was emphasized in a couple of Indo-European daughter languages, and so it was with Germanic that the perfect came to be used as a simple past tense. The semantic justification for this change is that actions of stative verbs generally have an implied prior inception. An example of this is the typical and widespread PIE stative *woida 'I know': one who "knows" something at some point in the past "came to know" it, much as the natural inference from noting someone in a sitting state is that a prior action of becoming seated occurred. The classical/Koine Greek perfect is essentially an early step in the development of the stative aspect to a past tense, being a hybrid of the two that emphasizes the ongoing effects of a past action. Apparently it was this latter anterior respect that is responsible for the Indo-European perfect showing up as a past tense in Germanic, Italic, and Celtic.
The Indo-European perfect took o-grade in the singular and zero grade in the dual and plural. The Germanic strong preterite shows the expected Germanic development of short o to short a in the singular and zero grade in the plural; these make up the second and third principal parts of the strong verb. The Indo-European perfect originally carried its own set of personal endings, the remnants of which are seen in the Germanic strong preterite. The reduplication characteristic of the Indo-European perfect remains in a number of verbs, a distinction by which they are grouped together as the seventh class of Germanic strong verbs.

Weak verbs

Weak verbs are those that use a dental suffix in the past or "preterite" tense, either or. In Proto-Germanic, such verbs had no ablaut—that is, all forms of all tenses were formed from the same stem, with no vowel alternations within the stem. This meant that weak verbs were "simpler" to form, and as a result strong verbs gradually ceased to be productive. Already in the earliest attested Germanic languages, strong verbs had become essentially closed classes and almost all new verbs were formed using one of the weak conjugations. This pattern later repeated itself—further sound changes meant that stem alternations appeared in some weak classes in some daughter languages, and these classes generally became unproductive. This happened, for example, in all of the West Germanic languages besides Old High German, where umlaut produced stem alternations in Class III weak verbs and, as a result, the class became unproductive and most of its verbs were transferred to other classes. Later, in Middle English, stem alternations between long and short vowels appeared in Class I weak verbs, and the class in its turn became unproductive, leaving the original Class II as the only productive verb class in Modern English.
In Proto-Germanic, there were five main classes of weak verbs:

Class I verbs were formed with a suffix -j-, e.g., Gothic satjan "to set", sandjan "to send", sōkjan "to seek". As shown in the Old English cognates, the -j- produced umlaut of the stem vowel in languages other than Gothic and then disappeared in most verbs in old Germanic languages other than Gothic and Old Saxon.
Class II verbs were formed with a suffix -ō-, e.g., Gothic salbōn "to anoint", Old English sealfian ← *salbōjan, compare "to salve".
Class III stative verbs were formed with a suffix that was -ja- or -ai- in the present and was null in the past, e.g., Old English hebban "to have" ← *habjan, past tense iċ hæfde "I had". The West Germanic languages outside of Old High German preserved this conjugation best, but in these languages the conjugation had become vestigial and had only four verbs in it. In other languages, it was merged with the Class III factitive verbs and significantly modified, e.g., Gothic haban, past tense ik habáida; Old High German habēn, past tense ih habēta.
Class III factitive verbs were formed with a suffix that was -ā- or -ai- in the present and -a- in the past. This class merged with the Class III stative verbs in Gothic, Old High German and Old Norse, but vanished in the other Germanic languages.
Class IV verbs were formed with a suffix -n-, e.g. Gothic fullnan "to become full", past tense ik fullnōda. This class vanished in other Germanic languages; however, a significant number of cognate verbs appear as Class II verbs in Old Norse and as Class III verbs in Old High German.
Preterite-presents

The so-called preterite-present verbs are a small group of anomalous verbs in the Germanic languages in which the present tense shows the form of the strong preterite. The preterite of the preterite-present verbs is weak. As an example, take the third-person forms of modern German können "to be able to". Kann "can, am/is able to" displays the vowel change and lack of a personal ending that would otherwise mark a strong preterite. Konnte "could, was able to" displays the dental suffix of the weak preterites.

Preterite-presents in Proto-Germanic

The known verbs in Proto-Germanic :

Infinitive	Meaning	Present verbal descendants	Class	Present singular	Present plural	Preterite
*witaną	"know"	Dutch weten, German wissen, Swedish veta, Norwegian Nynorsk vita, English wit	I	wait	witun	wissē
*lizaną	"know"		I	lais	lizun	listē
*aiganą	"have", "own"	Dutch eigenen, German eignen, Swedish äga, Norwegian Bokmål eie, Norwegian Nynorsk eige, English owe and ought	I	aih	aigun	aihtē
*duganą	"be useful"	Dutch deugen/gedogen, German taugen, Swedish duga, Norwegian duga, English dow	II	daug	dugun	duhtē
*unnaną	"grant"	Dutch gunnen, German gönnen, Swedish unna, Norwegian unna	III	ann	unnun	unþē
*kunnaną	"know ", later "can"	Dutch kunnen, German können, Norwegian kunne, Swedish kunna, English can	III	kann	kunnun	kunþē
*þurbaną	"need"	Dutch durven, German dürfen, Norwegian Nynorsk turva, Swedish torva/''tarva, English tharf	III	þarf	þurbun	þurftē
*durzaną	"dare"	Norwegian Nynorsk tora, English dare	III	dars	durzun	durstē
*skulaną	"must", later "shall"	Dutch zullen, German sollen, Norwegian Nynorsk skulla, Swedish skola, English shall	IV	skal	skulun	skuldē
	"think"	Icelandic muna, Norwegian Nynorsk muna, English northern dialectal mun	IV	man	munun
*ganuganą	"be enough", "suffice"	German genügen	V	ganah	ganugun	ganuhtē
*maganą	"can", later "may"	Dutch mogen, German mögen, Swedish må, English may	VI	mag	magun	mahtē
*aganą	"fear"	Norwegian Nynorsk age	VI	ag	ōgun	ahtē
*mōtaną	"may", later "must"	Dutch moeten, German müssen, Swedish måsta, English mote and must''	VI	mōt	mōtun	mōsē