Commas are used before the relative pronouns which or who
only in non-identifying relative clauses where the information
given is clearly extra to that given in the main clause.
In identifying relative clauses, where the clauses are a necessary
part of the nouns they describe, they are not used.
2. I think I remember that in reported speech in German, commas
are used before noun clauses that begin with that, what,
where, when, etc. In English they are not:
- 'Sarah, who came on holiday with me to Switzerland last
summer, is going to study anthropology at the University of Manchester.'
- 'This book, which was discovered in the ruins of a farmhouse in
Wales, is over four hundred years old.'
- 'The boy who introduced
me to the man I married has now married my cousin.'
- 'The book that
he discovered in the farmhouse in Wales is now worth a lot of money.'
- 'I had no
idea where to find the Ministry of Sound nightclub.'
- 'Let me know
what you plan to do when you get to London.'
- 'I wondered whether I
would get an invitation to Hugo and Sally's wedding.'
- 'He maintained
that he was innocent, but I knew that he was guilty.'
Note that co-ordinate clauses connected with and, but
or or (see above) are usually separated by commas.
In conditional sentences, they are used after the if clause
when the subordinate clause comes first, but if it comes second,
they are not always used.
4. They are used to separate items
in a list, but not between the last two items:
'If Henry rings, tell him I'm not at home.'
- 'I decided I would not
answer the phone if he rang.'
5. Adverbial clauses, phrases and expressions
are separated by commas, particularly when they are placed at the
beginning of sentences. As is the case in all of these examples,
they reflect pauses in speech:
- 'As a student, I
am always short of money, so when I go to the supermarket, I can
never afford to buy mangoes, grapefruit, steak, veal, ice cream
or ready-made meals.'
- 'Easter week, as everybody knows, changes from year to year.
This year it is very late.'
- 'In my view, he is innocent. However, in the jury's opinion,
he is guilty.'