The German modal verbs are a group of six verbs which affect the mood of a sentence, approximating to words like "can", "must" and "will" in English. Their combinations with German infinitive and their broad range of usage are discussed in Chapter 8 of this course.
In the present tense, the modal verbs:
have a zero ending on the "er/sie/es" form of the verb, i.e. there is no final -t
have a zero ending on the "ich" form of the verb, i.e. there is no final -e
apply any change to the stem vowel to the "ich" form of the verb, as well as to the "du" and "er/sie/es" forms. (N.B. The modal verb "sollen" does not modify its stem vowel at all.)
