Position of prepositions in relative clauses

To finish off the topic of relative clauses we need to take a look at one more 'problem' connected with them. Namely- the position of prepositions in those types of clauses. 

'The armchair in which the dog is lying is brown.' OR 'The armchair the dog is lying in is brown.'

Basically, prepositions can come either before relative pronouns (this is definitely MORE FORMAL!) or at the end of a relative clause (this is considered INFORMAL!):

The girl with whom I was sitting was very friendly. (FORMAL)

or

The girl (who) I was sitting with was very friendly. (INFORMAL)

This is the place in which I met her. (FORMAL)

or

This is the place (which) I met her in. (INFORMAL)

Pay attention to the fact that when the preposition is positioned at the end of a relative clause, the relative pronoun (who/which/that) is often omitted!

Remember that since WHO and THAT are less formal (than WHOM for example), we do not put a preposition before them:

The man with whom he works.

The man with who he works.

or

The man he works with.

Some more examples:

That's the house I told you about.

Do you remember the girl I was going out with in high school?

She's the only woman I've been in love with.

That's the reason I came here for.