Page 518 - Practical English Usage 3ed - Michael Swan, Oxford

Note that the conjunction (the second that) is usually dropped in this structure; it must be dropped if the relative pronoun is a subject.
This is the woman (who/that) Ann said could show us the church. (NOT This is the ftI6/'ltan (tt:Jh6lthat) Ann Sditi #tat etJl:lltl shew I:lS •••)
In this structure, people sometimes use whom as a subject pronoun. This is not generally considered correct.
Thisisaletterfrommyfather, whomwehopewiUbeoutofhospitalsoon. (More correct: ... who we hope will be out . ..)
Relative clauses can also be combined with if-clauses in sentences like the following.
I am enclosing an application form, which I should be grateful ifyou would sign and return.
16 a car that I didn't know how fast it could go, etc
We do not usually combine a relative clause with an indirect question structure. However, this sometimes happens in informal speech.
I've just been to see an old friend that I'm not sure when I'm going to see again.
There's a pile of washing-up that I just don't know how I'm going to do.
There is no grammatically correct way of doing this when the relative pronoun is the subject of the relative clause. However, sentences like the following (with added pronouns) are also sometimes heard in informal speech. Some real examples:
I was driving a car that I didn't know how fast it could go.
It's ridiculous to sing songs that you don't know what they mean. There's a control at the back that I don't understand how it works. 1here's still one kid that I must find out whether she's coming to the
party or not.
17 omission of subject
In a very informal style, a subject relative pronoun is sometimes dropped after
there is.
There's a man at the door wants to talk to you.
18 double object
Occasionally a relative pronoun acts as the object of two verbs. This happens especially when a relative clause is followed by before . . .ing, after . . .ing or without . . .ing.
We have water that it's best not to drink before boiling. (OR ••• boiling it.) I'm sending you a letter that I want you to destroy after reading. (OR • • • after
reading it.)
He was somebody that you could like without admiring. (OR ••• admiring
him.)
relatives (5): advanced points 498
page 486