Irregular verbs (also called “strong” verbs) change their root form as they are conjugated. For the most part, they form their present tense in exactly the same way as regular verbs. Thus “he swims” is er schwimmt, “they swim” sie schwimmen.
Some irregular verbs, however, will undergo a change in the stem vowel in the present tense singular, second and third person, for example: du gibst (you give) and er gibt (he gives) are conjugations of geben (to give). The importance of this change to the reader of German is that you will have to recognize that the meaning of, for example, gibst, will be found under the dictionary entry for geben. You should remember that there are four patterns of vowel changes in case you need to look up a verb in the dictionary:
infinitive > 3rd person sing.
|geben > gibt||e > i|
|stehlen > stiehlt||e > ie|
|halten > hält||a > ä|
|laufen > läuft||au > äu|
A list of the most common irregular verbs (strong verbs) is included in most dictionaries and grammar books. You do not have to memorize all the verb changes for reading purposes. The present tense singular, both second and third person, of these verbs will still carry the endings described above for weak verbs, ending in –st or –t.
There is one notable exception: the modal verbs, which are covered in Unit 10, and the verb wissen (to know a fact). The latter is conjugated in the present tense as: