Relative Adverbs

Relative adverbs are words like where, when and why. Although they are adverbs, they are used to introduce adjective clauses.

Where

The relative adverb where is used to introduce an adjective clause that modifies a noun of place. It is used in the same way as preposition + which.

She took me to a restaurant where they sell mediocre Chinese food. (= She took me to a restaurant at which they sell mediocre Chinese food.)
Here the relative adverb where modifies the verb sell, but the entire clause where they sell mediocre Chinese food modifies the noun restaurant.
Do you know a shop where I can get my watch repaired? (= Do you know a shop at which I can get my watch repaired?)
The house where I live is small. (= The house in which I live in is very small.)
My entire family still lives in the house where my great grandfather was born in 1895.

When can introduce relative clauses after nouns referring to time. It is used in the same way as preposition + which.

I will never forget the day when my daughter was born. (= I will never forget the day on which my daughter was born.)
My favourite month is March, when we celebrate Holi. (= My favourite month is March, in which we celebrate Holi.)
They have been waiting for that day when she will return home.

After common time expressions, when is often replaced by that.

Come and see me any time when you are in town. OR Come and see me any time that you are in town.
That was the year when I first went abroad. OR That was the first year that I first went abroad.

The relative adverb why can be used to modify the noun reason.

Do you know the reason why she doesn't like me? (= Do you know the reason for which she doesn't like me.)
I don't know the reason why she hasn't come yet.
The reason why I came here was to see my old classmates.
The reason why he lost his job was that he was dishonest.

Many writers now prefer that to why.

In an informal style, why / that is often left out.