DENKEN AN, but not denken auf, and also WARTEN AUF, but not warten an "Ich denke oft an dich." "Ich warte auf einen Brief." I simply don't understand and it looks like, it's not simple and beside that I'm not sure, if the prepositions in both sentences above are really correct. In both sentences we have Akkusativ, but the preposition in the first sentence is "an" and in the other "auf". Can we simply, say that only pairs "denken an" and "warten auf" are correct, but "denken auf" and "warten an" not.