Were you taught that a preposition should never be placed at the end of a sentence? There are times when it would be rather awkward to organize a sentence in a way that would avoid doing this, for example:
- in some passive expressions:
√ The dress had not even been paid for.
X Paid for the dress had not even been.
√ The match was rained off.
X Rained off was the match.
√ What did you put that there for?
X For what [reason] did you put that there?
X Of the commitment they are taking on they must be convinced.