Relative Clauses

Question

When I was working relative clauses, I was encountered by a footnote, and it sounds me weird.

FootNote: The objects or subjects described by relative clauses cannot take demonstrative pronouns or adjectives.

example: This theatre which exhibits the boring life of a rich man is very expensive.

Continue of footnote: The demonstrative pronoun "this" cannot be used if the noun is described with a relative clause. Instead of the word "this", the article "the" must be used.

Is it really true ?

why cannot we use "this" ?

Answer 1

The difference is between integral and non-integral, restrictive and non-restrictive clauses.

If the clause is non-integral/non-restrictive, it is merely a bit of info about the noun.

This hat, which has three corners, is her favorite.

The hat happens to have three corners.

If the clause is integral/restrictive, it specifies which noun you're talking about.

The hat which has three corners is her favorite. Not the round one.

If the clause does the specifying, you cannot also specify the noun by pointing at the noun with this. If you use this, you're effectively casting the clause as an extra bit of info, not as a specifier:

This red hat, which has three corners, is her favorite. Not the blue one.
-- OK, but why are you telling me about the corners? Are they relevant?

Here's another example with that:

You can sleep in the room that has the king-sized bed.

You can sleep in this room that has the king-sized bed. ^{ungrammatical}