There were about three hundred strong verbs in OE. They were native words descending from PG with parallels in other OG languages; many of them had a high frequency of occurrence and were basic items of the vocabulary widely used in word derivation and word compounding.
The strong verbs in OE (as well as in other OG languages) are usually divided into seven classes.
Classes from 1 to 6 use vowel gradation which goes back to the IE ablaut-series modified in different phonetic conditions in accordance with PG and Early OE sound changes. Class 7 includes reduplicating verbs, which originally built their past forms by means of repeating the root-morpheme; this doubled root gave rise to a specific kind of root-vowel interchange.
As seen from the table ↓the principal forms of all the strong verbs have the same endings irrespective of class: -an for the Infinitive, no ending in the Past sg stem, -on in the form of Past pl, -en for Participle II. Two of these markers – the zero-ending in the second stem and -en in Participle II – are found only in strong verbs and should be noted as their specific characteristics. The classes differ in the series of root-vowels used to distinguish the four stems. However, only several classes and subclasses make a distinction between four vowels as markers of the four stems – see Class 2, 3b and c, 4 and 5b; some classes distinguish only three grades of ablaut and consequently have the same root vowel in two stems out of four (Class 1, 3a, 5a); two classes, 6 and 7, use only two vowels in their gradation series.
In addition to vowel gradation some verbs with the root ending in -s, -p or -f employed an interchange of consonants: [s~z~r]; [θ~ð~d] and [f~v]. These interchanges were either instances of positional variation of fricative consonants in OE or relics of earlier positional sound changes; they were of no significance as grammatical markers and disappeared due to levelling by analogy towards the end of OE.
The classes of strong verbs – like the morphological classes of nouns – differed in the number of verbs and, consequently, in their role and weight in the language.