Aanbieden is a separable verb, it means to offer. Bieden also is a verb, meaning to bid/to make an offer. It's a bit annoying in this case that both are related to different meanings of offering in English. But what I mean to say is that aanbieden and bieden simply are different words with different meanings. So the aan in the sentence is part of the verb, it is not a preposition.
The part that separates from the rest of the verb, usually goes to the end of the sentence.
If you want to know more about separable verbs, you can find that here. In the bottom right there is a whole menu explaining about compount verbs (separable verbs are compound verbs…namely the ones you can separate…) :)
Probably not the clearest explanation, but you could see it like this: often separable verbs are verbs which have sort of 'absorbed' a preposition or another short and common word (so the preposition becomes part of the verb and it not a preposition anymore), and a new verb is born. An example:
- geven - to give
- opgeven - to give up
- overgeven - to throw up
- afgeven - to stain
- aangeven - to hand over/to indicate
The downside of this is that they can be a bit hard to recognise, and there are a lot of them…one example that shows why this can be difficult for Dutch learners:
- Ik geef het boek aan jou. = I give the book to you. Verb = geven.
- Ik geef het boek aan. = I hand over the book. Verb = aangeven.
- Ik geef het boek aan jou aan. = I hand over the book to you. Verb = aangeven.
Haha, honestly I thought that's what it was but since "bieden" translates (according to Google, at least) to "offer", I was just making sure it was required. I definitely know what seperable verbs are, because German has lots of these as well. Thank you for taking the time to thoroughly explain it, though. I really do appreciate all the help you've given me. :)