So we can use relative clauses to join two English sentences, or to give more information about something.
I bought a new car. It is very fast. → I bought a new car that is very fast.
She lives in New York. New York is a wonderful city. → She lives in New York, which is a wonderful city.
After studying the different relatives we have to bear in mind there are two types of relative sentences:
Defining Relative Clauses
Defining relative clauses give detailed information defining a general term or expression. Defining relative clauses are not put in commas. Defining relative clauses are often used in definitions, e.g.. A seaman is someone who works on a ship.
Object pronouns in defining relative clauses can be dropped, e.g. The boy (who/whom) we met yesterday is very nice.
Not sure when you can drop the pronoun? Have some practice on this quiz
Non-Defining Relative Clauses
Non-defining relative clauses give additional information on something, but do not define it. Non-defining relative clauses are put in commas, e.g. Jim, who/whom we met yesterday, is very nice.
Note: In non-defining relative clauses, who/which may not be replaced with that.
Now a mix of them all: 1