Frank van den Eeden

I don't remember them exactly, but this is what I found :
THAT omission

Relative pronoun :
may be suppressed in a restrictive clause provided it does not serve as the subject of the main verb.
ex.: Elephants that love mice are very unusual.
(You can’t drop “that” here)


None-defining relative clauses :
The information in these clauses is not essential. It tells us more about someone or something, but it does not help us to identify them or it.
ex.: Elephants, which are large and grey, can sometimes be found in zoos. (This gives us some extra information about elephants - we are talking about all elephants, not just one type or group).

see the difference with : Elephants that love mice are very unusual. (This tells us which elephants we are talking about). (here you cannot leave “that” out because “the elephants” is subject).

1. In non-defining clauses, you cannot use ‘that’ instead of who, whom or which.
2. You cannot leave out the relative pronoun, even when it is the object of the verb in the relative clause:
He gave me the letter, which was in a blue envelope.
He gave me the letter, which I read immediately

Punctuation
Non-defining relative clauses are always separated from the rest of the sentence by commas. The commas have a similar function to brackets:
My friend John has just written a best-selling novel. (He went to the same school as me) > My friend John, who went to the same school as me, has just written a best-selling novel.


That gives you something to start with !
But, as usual, the theory makes it seem more difficult than it really is.
Anyway, as long as you're not sure, follow Vicente's advice.


PS. omission

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