Remember

1. In non-defining relative clauses you can use which to refer to the whole clause before. We have to use a comma before which.

• Most of the students passed the exam, which made the teacher very proud.

2. We use that (not which) after:

→ something, everything, nothing, anything
→ numeral
→ all, some, any, no, none, little, few
→ the superlative

• Everything (that) he said was true.
• The first thing (that) you must do is marry her.
• All visitors (that) come to Prague admire it.

3. What is not a relative pronoun. It is at the beginning of a noun-clause, or an object-clause.

• Show me what you have in your hand.          X           Show me everything that you have in your hand.
• What he said was true.          X           Everything (that) he said was true.

4. Sentences with two meanings:

• My brother who is in Switzerland is a surgeon.
  → I have another brother or other brothers somewhere else.
• My brother, who is in Switzerland, is a surgeon.
  → I have only one brother.

• The test paper which everyone failed was too difficult.
  → There were other papers, which everyone did not fail.
• The test paper, which everyone failed, was too difficult. 
  → There was a single test paper and everybody failed it.

5. Quantity phrase pronouns:

→  both of whom
→  none of whom
→  all of whom
→  few of whom
→  a lot of whom
→  nothing of which
→  neither of which 

• They have two sons, both of whom are tall.
• Three buses arrived, all of which were crowded.
• She showed me two coats, neither of which fitted me.
• They’ve built twenty flats, few of which have two bathrooms.
• There were five boys at the party, none of whom she liked.