1. The relative pronoun as subject
1.1. When the relative pronoun is subject of a clause and refers to a human, the relative pronoun who is generally used. :
I know someone who eats red hot chilli peppers.
Sometimes, who is replaced by that, especially in American English and in spoken language:
is also quite possible.
After the antecedent those, who is almost always required:
1.2. If the relative is the subject of a clause and refers to an inanimate antecedent, which or that must be used.
The book which is on the table is mine.
Omission: As subject of a clause, the relative pronoun can never be omitted. However, the relative clause can be completely omitted:
The book on the table is mine is perfectly acceptable.
When the relative pronoun is the direct object of the clause, and refers to a human, the pronoun used is either whom or that.
The man that I saw yesterday is 99.
Alternatively, the relative can be omitted, particularly in spoken language:
The man I saw yesterday is 99.
Whom is not used very often: that, or omission of the relative pronoun, are much more common.
When an inanimate object is referred to, the same rules apply, except that whom is never used: it is replaced by which.
The book which I was reading was very interesting, or
The book I was reading was very interesting
are all possible
Omission: when it is the object of the relative clause, the relative pronoun can often be omitted, particularly in written English.
Whose is required with both animate and inanimate antecedents: it is the only derivative of who which can refer to animates and inanimates:
The man whose car I borrowed is very rich.
I chose the set whose price was reduced.
4.1. Note how to form relative clauses after prepositions: preposition+which for inanimates or things, preposition + whom for people. Stylistically, this is quite formal.
The chair on which I sat down collapsed.
4.2. If the relative pronoun is omitted, then the proposition must come at the end of the clause. Omission of the relative pronoun in examples like the ones below is actually by far the most common usage in modern spoken English, and is also common in written style.
The chair I sat down on collapsed.
5.1 Preposition + possession:
It is to my parents, thanks to whose generosity I was able to complete my studies, that I am most grateful.
5.2. Selective possession
The writer, the first of whose books had been a bestseller, was a coal miner.
There are several ways to go from London to Scotland , the fastest of which is of course by plane.
6.1. A “Defining” relative clause is one which is essential for the understanding of a statement.
In this example, it is clear that “all protestors who smash windows” will be arrested. The word “protestors” in this example is restricted by the relative clause that defines it
Commas are not required before and after the relative clause.
6.2. In a non-defining relative clause, the relative clause is not essential for an understanding of the sentence:
In this example, the question of age is not an essential bit of information. The relative clause can be omitted without making the sentence meaningless.
In cases like this, commas are usually required before and after the relative clause.
Compare these two examples:
6.2. Sportsmen, who pay attention to their diet, are not usually over-weight.
Sometimes we use a relative clause to qualify not just a noun or pronoun, but a whole sentence or clause. In such cases, the relative clause is introduced by which, never
that or what.
It was raining yesterday, which was a pity.
There aren’t enough tables in the exam room, which is rather a problem.
This point is dealt with above in the sections 2, 3 and 4 above.
Note in particular the question of omitting the relative pronoun in a prepositional relative clause (point 4).
English grammar books sometimes say that it is bad style to end a sentence with a preposition; but this is just not true. On the contrary, when the relative pronoun is omitted in a prepositional relative clause, the preposition must come at the end of the clause, even if this is also the end of the sentence. As stated above, omission of the relative pronoun in prepositional relative clauses is normal style in modern English.
Our company currently has enough financial reserves to get by with.
The project our team is currently working on is of huge potential significance.