Relative pronouns are used to join clauses (like conjunctions) and, at the same time, to replace the subject or object of verbs (like other pronouns).
Who – Used for people. Refers to the subject.
A gardener is a person. He takes care of plants and flowers.
A gardener is a person who takes care of plants and flowers
Which – Used for things and animals. Refers to the subject or object.
The cat eats meat every day. It lives in the house.
The cat which lives in the house eats meat every day.
The perfume was very expensive. She bought it in France.
The perfume which she bought in France was very expensive.
When which refers to the object of the sentence it can be left out.
The perfume she bought in France was very expensive.
That – Used for things, people and animals.
The perfume that she bought in France was very expensive.
A gardener is a person that takes care of plants and flowers
The cat that lives in the house eats meat every day.
When that refers to the object it can be left out.
The perfume she bought in France was very expensive.
Whose – Used for things, people and animals. Refers to possession.
He read a book whose author was Sidney Sheldon.
Whom – Used for people as the object of a verb or preposition. Refers to the object (formal). It is frequently replaced by who, that or can be left out (0).
He married a pretty woman. He got to know in Italy.
He married a pretty woman whom, who, that or 0 he got to know in Italy.
The girl left him after few weeks. He fell in love with the girl.
The girl with whom he fell in love left him after a few weeks.
(fell in love exige with e se deslocar a preposição, só pode usar whom)
The girl whom/who he fell in love with left him after a few weeks.
(Neste caso o with não foi deslocado portanto pode-se usar whom or who)
John is one of my closest friends. I have known John for eight years.
John, who/whom I have known for eight years, is one of my closest friends.
Omission Of Relative Pronouns
Relative pronouns can be omitted when they are not the subject of the relative clause.
1) The girl is gorgeous. I love her.
The girl Who(m)/that I love is gorgeous.
The girl I love is gorgeous
2) The girl is gorgeous. She loves me.
The girl who loves me is gorgeous.
The omission is possible in sentence number 1 because “I” is the subject of the relative clause, whereas in the sentence number 2 the relative pronoun is the subject.
TIP: se depois do relative pronoun ter um pronome pessoal ou nome pode omitir, mas se tiver um verbo não.