When do I need ‘zu’ before a verb?
Can’t think of any examples sorry about that.
Here is an example for you:
Auch der kleinste Feind ist nicht zu verachten. - German proverb
(Even the smallest enemy is not to be despised.)
Here's a link with some information and more examples:
Wer "brauchen" ohne "zu" gebraucht, braucht "brauchen" gar nicht zu gebrauchen.
there are words which just trigger the infinitive clause, such as versuchen (to try) Ich versuche mein Handy zu finden.
The infinitive clause is also triggered because the verb in another clause has context in the main clause, if that makes sense. Such as an example given from volga, 'auch der kleinste Feind, ist nicht zu verachten. Also the smallest enemy, is the main clause, then we go to 'is not to despise' in this clause their is no subject, but in the first one there is, the enemy. so since the subject is in only the first clause we use the infinitive clause to show that.
That is my understanding of it, there are trigger words like versuchen and clause things. here is video https://www.youtube.com/watch?v=HwxJilMaCuU