From Sue O’Connell Focus on Proficiency
Defining relative:
1. Commas are not used to separate the relative clause from the rest of the sentence.
2. ‘that’ can be used instead of who or which
Eg The man that you wanted to see is on the 'phone.
It's a problem that has taxed many brains.
3. Where the relative pronoun is the object, it can be omitted, as in sentence.
Eg The car she wanted x has been sold.
4. ‘who’ and ‘which’ are never used as object pronouns after superlatives, time expressions or indefinite pronouns.
They are either omitted or replaced by ‘that’
Eg This is the most expensive present (that) I ever intended to buy.
I met him on the day (that) I was due to leave.
Choose a seat anywhere (that) you want.
5. The possessive pronoun for both people and things is ‘whose’
Eg He didn't seem like a man whose friends had all deserted him.
We stayed in a hotel whose only recommendation was its cheapness.
Non-defining
1. The non-defining relative clause must be separated from the rest of the sentence by commas, as in:
Jean Bond, who is running the special unit while on sabbatical from Warwick University's
education department, says...
Or occasionally by brackets as in:
The younger children read to their tutors (who are supervised by university
students of education)...
2. ‘that’ cannot be used to replace who which whom
3. The relative pronoun cannot be omitted.
FROM: http://linguapress.com/grammar/relative-clauses.htm
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. Examples:
The man who lives next door is 99.
I know someone who eats red hot chilli peppers.
Sometimes, who is replaced by that, especially in American English and in spoken language:
The boy that lost his watch was careless.
However,
The boy who lost his watch was careless.
is also quite possible.
After the antecedent those, who is almost always required:
Those who can swim should go first.
