Relative cleuses "that" or "about which"

English as a Second Language English Vocabulary, Grammar and Idioms
1

Relative clauses - “that” or “about which”
In the sentence

“It’'s a subject ___________ I know a great deal”

Which one is correct:

“that” ot “about which” ?

and Why ?

Again thanks in advance

2

Hi,

You can’t use ‘that’ with a preposition - before it. It is also possible to say: It’s a subject that I know a great deal about.

Alan

3

<which, who, that = conj+pron>

1.I love the girl [color=red]who is standing there.
= I love the girl [color=green]and she is standing there.
(You can tear who into a conj and a pron)

2.I love the boy [color=red]who/whom you’re talking to.
= I love the boy to whom you’re talking. (after a prep it’s better not use ‘who’)
= I love the boy that you’re talking to.
= I love the boy [color=green]and you’re talking to [color=green]him.
(the pron from who can be the subject and the object)

As for your sentence:

=It’s a subject [color=red]and I know a great deal about [color=red]it/the subject.
=It’s a subject [color=green]that I know a great deal about.
=It’s a subject [color=green]which I know a great deal about.
=It’s a subject about[color=green] which I know a great deal.

[color=red]not

It’s a subject about[color=green] that I know a great deal.
or
It’s a subject [color=green]that I know a great deal.
(As the clause needs a prep ‘about’)