A relative clause often starts with a word that isn’t a relative pronoun. This is usually either a relative adverb –when, where, or why– or a relative adjective –which or whose.
The climate is the reason why we moved to California.
In 1948, when revolutions broke out across Europe, marx published his communist Manifiesto.
They had been to Mexico’s Cooper Canyon, where the Taraumara Indians live.
My favoroite uncle, whose jokes were funnier than anyone else’s gave great parties.
It’s not clear which road we’re supposed to take.
That was teh year when he died (Or: That was teh year in which he died).
That is the house where I was born (Or: That is the house in which I was born.)
Note that when and where can often be replaced by “in which” if the stated time or place (“the year”, “the house”) could have the word “in” in front of it.
When and why can often be omitted from a defining clause:
The day (when) the festival was held was cloudy and cold.
That is the main reason (why) the war lasted so long.
Where may also often be omitted, but only if a preposition is added.
That is the town where I grew up. = That is the town I grew up in.
The verb in a relative clause must always agree in number with the noun or pronoun that is being modified. That is, both noun (or pronoun) and verb must be singular, or both must be plural:
The land that they own is (not are) now very valuable.
My brothers, who are (not is) very conservative, prefer the other candidate.
The numbers in the document, which were checked by my assistant, are all accurate.
Note that the noun or pronoun that is modified is not always the last noun before the relative clause.