The subject of history of English

20. Classes of Old English Verbs as Reflected in present-day English Verb Forms.
Old English verbs can be daunting, for a typical verb appears in more forms than a typical pronoun, noun or adjective. The Old English weak verbs correspond roughly to the Modern English “regular” verbs. Helpan ‘help’ is a “strong” verb, one that does not add a dental suffix to make its past tense, but rather changes the vowel of its root syllable. The Old English strong verbs correspond to Modern English “irregular” verbs such as sing (past sang, past participle sung).

There are just two tenses, past and present. Old English has various strategies for referring to future time: it uses auxiliary verbs (including willan), explicit references to time (e.g. tōmorgen ‘tomorrow’), and the simple present, relying on context to express futurity.

Similarly, Old English has no settled way of expressing what Modern English expresses with the perfect and pluperfect—that is, that an action is now complete or was complete at some time in the past. It can use forms of the verb habban ‘to have’ with the past participle, as Modern English does (hæfð onfunden ‘has discovered’, hæfde onfunden ‘had discovered’), it can use the adverb ǣr ‘before’ with the simple past (ǣr onfand ‘had discovered’), or it can use the past tense alone, in which case you must infer the correct translation from the context.

Some of the Modern English auxiliary verbs (also called “helping verbs”) are descended from a class of Old English verbs called “preterite-presents.” They are so called because the present tense of these verbs looks like the past tense (what many grammar books call the “preterite”) of the strong verbs. Most of these Modern English preterite-presents come in pairs, one member of which was originally a present tense and the other originally past: can/could, may/might, and shall/should.

Modern English makes a distinction between regular and irregular verbs. This distinction goes back to the Old English system of strong and weak verbs: the ones which used the ancient Germanic type of conjugation (the Ablaut), and the ones which just added endings to their past and participle forms. Strong verbs make the clear majority. According to the traditional division, which is taken form Gothic and is accepted by modern linguistics, all strong verbs are distinguished between seven classes, each having its peculiarities in conjugation and in the stem structure. It is easy to define which verb is which class, so you will not swear trying to identify the type of conjugation of this or that verb (unlike the situation with the substantives).

Examining verbs of Old English comparing to those of Modern English it is easy to catch the point of transformation. Not only the ending -an in the infinitive has dropped, but the stems were subject to many changes some of which are not hard to find. For example, the long í in the stem gives i with an open syllable in the modern language (wrítan > write, scínan > shine). The same can be said about a, which nowadays is a in open syllables pronounced [æ] (hladan > lade). The initial combination sc turns to sh; the open e was transformed into ea practically everywhere (sprecan > speak, tredan > tread, etc.). Such laws of transformation which you can gather into a small table help to recreate the Old word from a Modern English one in case you do not have a dictionary in hand, and therefore are important for reconstruction of the languages.