I. TheoryThe preposition zu is often found before infinitives in German, much like “to” is in English. However, in German, zu is not present when there is a modal verb plus an infinitive. Therefore, zu is most frequently seen when the infinitive in question is in its own clause (an Infinitivsatz) or used in conjunction with a verb that is not a modal verb.
|Ich muss meine Hausaufgaben machen (no “zu” due to modal verb).|
I must do my homework.
|Ich habe vergessen, meine Hausaufgaben zu machen.|
I forgot to do my homework.
In the sentence above, we see th