The modern
English verb system is not something too hard to pin down. Little
inflection depending on person, few
irregulars, simple conjugation into past, progressive, and participles; it's all a pretty good deal. Things certainly weren't always that way.
Old English during its productive literary stage was a transitional language, moving from an ancient
Germanic system of conjugation by
ablaut to the more common
Indo-European system of conjugation by inflection. It wasn't completely done yet. About 20% of verbs (and a higher proportion of common verbs) one will encounter in an Old English text hadn't made the jump to regularized conjugation yet, they were called
strong verbs.
Strong verbs, as the name implies, followed a very different system of conjugation that couldn't easily be predicted, unlike their counterparts
weak verbs.
Strong Verb Classes
It is a poor consolation, but welcome one, that there are at least some sketchy rules. First it would be a good idea to get a grasp of the process of ablaut, however. In modern English we have a couple of verbs left that are still strong. Write, wrote, written - come, came, come - Speak, spoke, spoken etc. Looking at them, you'll notice a there's a pattern to their differentiation. The root vowel, the one onto which stress is placed, changes in the past tense form. This is called ablaut. Furthermore, these aren't isolated tendencies, there are certain categories of similar irregular verbs that all undergo ablaut the same way. For example swim, swam, swum - sing, sang, sung - perhaps one could even argue for C!ing, C!ang, C!ung. There are other functional categories that contain a few irregular verbs each. They don't work for everything, but sometimes a pattern can be found.
The same goes for Old English, only moreso. For strong verbs not entirely irregular, there were seven classes of conjugation. The variations displayed in some of these classes require four principal parts (unlike Old English weak verbs, or modern English verbs). These are the infinitive, past tense first-person singular, past tense plural, and the past participle. Even though four principle parts are necessary, that does not mean each class had four different vowels for each. Some had three, some had two.
Class 1
This class is characterized by a long vowel single consonant root syllable, specifically -íC where C is any singular cosonant. The root vowels and their changes go as follows.
Present: -í-
Past Sg.: -á-
Past Pl.: -i-
Past Part.: -i-
Examples of this class of verb are dífan, dráf, drifon, drifen (drive), rísan, rás, rison, risen (rise), bídan, bád, bidon, biden (abide).
Class 2
This class was actually characterized by two present tense vowel roots, -éo and -ú, that followed ablaut in exactly the same fashion. They are also of the -VC template. There is no way of telling whether a verb has a present tense form with -ú or with -éo from looking at its other ablauted forms; their roots of development go a very long way into ancient Indo-European history from different vowel grades.
Present: -éo-/-ú-
Past Sg.: -éa-
Past Pl.: -u-
Past Part.: -o-
Examples include dréopan, dréap, drupon, dropen (drip), slúpan, sléap, slupon, slopen (slip), and léogan, léag, lugon, logen (lie).
Class 3
Things looked pretty easy until now, didn't they? No longer. The third class of strong verbs is characterized by the template -VCC, where V is a short vowel. In distant times before Old English was written, the original pattern of ablaut was e, a, u, and u. Through process of vowel change, this class split into five different sub-categories . The linguistic phenomona behind these changes can be traced, for example category 1 illustrates the fronting of the short a in West Germanic languages, while category 3 undergoes a process called 'breaking' related to the presence of a syllabic r in the stem. Mostly though, one will just need to memorize the principle parts of strong class three verbs. Here are the subcategories.
Category 1
Present: -e-
Past Sg.: -æ-
Past Pl.: -u-
Past Part.: -o-
Category 2
Present: -e-
Past Sg.: -ea-
Past Pl.: -u-
Past Part.: -o-
Category 3
Present: -eor-
Past Sg.: -ear-
Past Pl.: -ur-
Past Part.: -ur-
Category 4
Present: -ie-
Past Sg.: -ea-
Past Pl.: -u-
Past Part.: -o-
Category 5
Present: -i-
Past Sg.: -a-
Past Pl.: -u-
Past Part.: -u-
Examples for all five categories are bregdan, bægd, brugdon, brogden (shake), helpan, healp, hulpon, holpen (help), beorcan, bearc, burcon, burcen (bark), gielpan, gealp, gulpon, golpen (boast), and drincan, dranc, druncon, druncen (drink).
Class 4
Class four strong verbs are characterized by a short -e- followed by a liquid consonant, either 'r' or 'l'.
Present: -e-
Past Sg.: -æ-
Past Pl.: -áé-
Past Part.: -o-
Examples include stelan, stæl, stáélon, stolen (steal) and beran, bær, báéron, boren (bare). There aren't many of them.
Class 5
Almost exactly the same as class four, except that the short -e- was followed by a different single consonant. The Past participle changed to -e-, in this case. Examples include sprecan, spæc, spéácon, sprecen (speak) and awrecan, awræc, awréácon, awrecen (recite). Again, not many of these.
Class 6
This class had a short -a- in the root syllable. Because of a tendency to to front the 'a' in West Germanic languages, the past participle form often took an -æ- (more natural than the -a-). By analogy with the infinitive, however, this was sometimes morphed back into an -a- again. In other words, just memorize the past participle form, it will cause less headaches.
Present: -a-
Past Sg.: -ó-
Past Pl.: -ó-
Past Part.: -a-/-æ-
Examples include wascan, wósc, wóscon, wascen (wash), bacan, bóc, bócon, bacen (bake), and faran, fór, fóron, færen (go).
Class 7
Regular root syllable? Predictable past participle? BWAHAHAhahahahaha..ha.. heh heh *cough*. Anything could be a class seven verb, no matter the root syllable. Memorization is your friend.
Present: you wish
Past Sg.: -é-
Past Pl.: -é-
Past Part.: start weeping now
Examples include láétan, lét, léton, láéten (let), feallan, féoll, féollon, feallen (fall), and bláwan, bléow, bléowon, bláwen (blow).
Strong Verb Conjugation
So long spent struggling with the principal parts, and you thought you'd get a rest? Hardly. Strong verbs followed their own rules of conjugation separate from weak verbs, relying on the principle parts so painstakingly derived above. Certain endings sometimes caused consonant changes to the root syllable; a dissertation could be written on this topic and influences, but for purposes of reading or writing you'll just need to memorize. I hope you're starting to hate strong verbs as passionately as I do right about now.
Present
1 ic | -e
2 þú | -est
3 hé/héo/hit | -eð
Pl. | -að
Past
Using the past tense principal part for singular and past tense plural principal part for plural, remember.
1 | -
2 | -e
3 | -
Pl. | -on
To all of these fragile rules there were plenty of exceptions. In practical terms, remembering all the classes right off the bat and making good use of them is nigh impossible. Only by the gradual process of memorizing each strong verb individually and recognizing patterns (as we do in modern English, modern German too, by the way) can the whole system really be grasped. But don't worry, it's only one out of every five verbs you'll see... *sob*