Basic Things: Strong Verbs

The verbal system is more archaic than that of most Germanic languages. There is a large number of strong (irregular) verbs, in addition to weak (regular) ones, which fall into two classes. Like in many other Germanic languages, all the infinitive forms end in -en. A certain group of weak verbs may have an -an infinitive in archaic speech.

Strong Verbs 
The strong verb class is mainly composed of verbs that do not end in -nen or -jen.
The inflectional paragidm of the strong verbs is as follows (the examples are given for the verb drinken 'to drink':

person	Present	Past	Future	example Present	example Past	example Future
1s (I)	-a	-0	-ou	ik drinka	ik drank	ik drinkou
2s (thou)	-s	-t	-ais	þu drinks	þu drankt	þu drinkais
3s (he/she/it)	-t	-0	-ai	is drinkt	is drank	is drinkai
1p (we)	-am	-u	-aim	vis drinkam	vis drunku	vis drinkaim
2p (you)	-t	-u	-ait	jus drinkt	jus drunku	jus drinkait
3p (they)	-en	-u	-ain	jai drinken	jai drunku	jai drinkain

The endings of 2s, 3s and 3p may have a prothetic -i- for euphonic purposes in case of two t's in a row or an exceding number of consonants in a root, for example, is vorstet 'he works' instead of *is vorstit.

The participles are derived in the following way:
Active: -nd, e.g. drinkend 'drinking'
Passive: -(e)t, e.g. drunkt 'drunk'

The most peculiar thing about the strong verbs is the Ablaut - there are different Ablaut paradigms for different root types:

Ia. Infinitive CiC-: Past singular CaiC, Past plural CiC-, Past participle CiC-
(biten 'to bite': ik bait 'I bit', jai bitu 'they bit', bitet 'bitten')
Ib. Infinitive Cix-/Cir-: Past singular Caix/Cair, Past plural Caixu/Cairu, Past participle Caix-/Cair-
(þixen 'to gain': ik þaix 'I gained', jai þaixu 'they gained', þaixet 'gained')
IIa. Infinitive CeuC-: Past singular CouC-, Past plural CuCu, Past participle CuC-
(geuten 'to pour': ik gout 'I poured', jai gutu 'they poured', gutet 'poured')
IIb. Infinitive Ceux-: Past singular Coux-, Past plural Couxu, Past participle Coux-
(teuxen 'to lead': ik toux 'I led', jai touxu 'they led', touxet 'led')
IIIa. Infinitive CiC-: Past singular CaC-, Past plural CeCu, Past participle CiC-
(giten 'to get': ik gat 'I got', jai getu 'they got', gitet 'got')
IIIb.  Infinitive CaiC-: Past singular CaC-, Past plural CeCu, Past participle CaiC-
(saipen 'to see': ik sap 'I saw', jai sepu 'they saw', saipet 'seen')
IVa. Infinitive CiNC-: Past singular CaNC-, Past plural CuNCu, Past participle CuNC-
(finden 'to find': ik fand 'I found', jai fundu 'they found', fundet 'found')
IVb. Infinitive CerC-: Past singular CarC-, Past plural CorCu, Past participle CorC-
(verpen 'to throw': ik varp 'I threw', jai vorpu 'they threw', vorpt 'thrown')
V. Infinitive CaC: Past singular CoC-, Past plural CoCu, Past participle CaC-
(graben 'to dig': ik grob 'I dug', jai grobu 'they dug', grabt 'dug')
VI. Infinitive CouC: Past singular CauC-, Past plural CuCu, Past participle CuC-
(galouzen 'to lose': ik galauz 'I lost', jai galuzu 'they lost', galuzet 'lost')

There are exceptions even from these rules. For example, the word keuden 'to say' is kad, kedu in the past (not *koud, kudu), and the word frexnen 'to ask; is frax, frexu, although it has a form similar to a weak verb. Some verbs that have a suitable form to be strong are really weak, e.g. liben 'to live'. Also, in the modern language there is no difference in the nature of the roots belonging to the classes Ia and IIIa. All this makes the overall system of strong verbs quite complicated; it should be noted, however, that many of the discrepancies can be explained by historic processes.