Stranding

Traditional grammatical rules say that we should not have a preposition at the end of a clause or sentence. However, we sometimes do separate a preposition from the words which follow it (its complement). This is called preposition stranding, and it is common in informal styles:

-       She was someone to whom he could talk. (formal)

-       She was someone who he could talk to. (informal)

-       Which room are they having breakfast in? (informal)

-       In which room are they having breakfast? (formal)

If we leave out words that are clear from the context (ellipsis), we can use wh-questions with a wh-word + stranded preposition:

A: The office is moving next year.

B: Really, where to?

A: I’m going to buy some flowers online.

B: Who for?

A: My mother.