To refer to things, use which and that as the subjects of verbs in adjective clauses.

**NOTE : That is less formal than who or which.

To refer to things, use which and that as the objects of verbs in adjective clauses. Which is a bit more formal.

In conversation and formal writing, you can sometimes omit the relative pronoun if it is an object. This is the most common spoken form.

The verb in an adjective clause agrees with the subject of the clause, not with the object.

You can use when or that to begin an adjective clause that modifies a noun of time. You can omit when and that in this type of adjective clause. A sentence without when or that is informal.

An adjective clause that adds extra information but does not distinguish one person or thing from another is called nonidentifying or nonessential. The clause is enclosed in commas.

Adjective: A word that describes a noun, such as the friendly dog or the tall fence.

Clause: Two or more words.

Basically, an adjective clause gives a description of a noun and often begins with a pronoun or an adverb. 

Who (My sister, who is older than me, is a great athlete.)

Whom / Whose (I know someone whose cousin is famous.)

That (The TV that is in the living room is broken.)

Which (Valencia, which is a type of orange, is great for making juice.)

When (Last Saturday, when it was raining, I fell off my bike.)

Where (The gym, where I go twice a week, will shut down soon.)

Why (Fear of heights is the main reason why my mother doesn't like tall buildings.)

Because an adjective clause does not express a complete thought, it is not a sentence. An adjective clause has to be connected to a main clause to form a complete sentence, as shown in this example: 

‘When I finish my homework, I often eat pizza.”

Adjective clause: when I finish my homework

Simple sentence / main clause: I often eat pizza