Some verbs need a preposition before an object or another verb. The preposition is only grammatical, it doesn't change the meaning of the verb.
The only way to know which preposition to use after the verb is to memorize the verbs with accompanying prepositions.
Welcome to Memrise!
Join millions of people who are already learning for free on Memrise! It’s fast, it’s fun and it’s mind-bogglingly effective.