Dutch verbs often combine with fixed prepositions (vaste voorzetsels) that cannot be logically predicted — they must be memorised as units. Common combinations: houden van (to love/like — van is always used), kijken naar (to look at — naar, not op), luisteren naar (to listen to — naar), wachten op (to wait for — op, not voor), denken aan (to think of/about — aan), denken over (to think about in the sense of considering — over).
More fixed combinations: praten over (to talk about), praten met (to talk with), vragen om (to ask for something), vragen naar (to ask about someone), zoeken naar (to search for), zorgen voor (to take care of), geloven in (to believe in), reageren op (to react to/respond to), bestaan uit (to consist of), lijden aan (to suffer from), stemmen op (to vote for), schrijven over (to write about).
Waar-compounds replace prepositional phrases when referring to things (not people): Waar denk je aan? (What are you thinking about? — literally: where are you thinking about?). Ik denk eraan (I am thinking about it — er replaces the object + preposition). Waar wacht je op? (What are you waiting for?). Ik wacht erop (I am waiting for it). For people, the preposition + pronoun stays: Ik denk aan hem (I am thinking about him). This er/waar system is one of the most typically Dutch grammatical features.