Could you please direct me to the grammar « topic » which would explain this concept in more detail? I am simply unable to find the topic in my grammar reference.
For example, in Michael Swan's Practical English Usage, Oxford University Press, 2017, page 357, section 237, we can read:
17 somebody I know you 'II like
It is often possible to combine relative clauses with indirect statements and
similar structures, e.g. I know/said/feel/hope/wish (that) . . ., especially in an informal style. Expressions like I know, I said, etc. come after the position of the
We’re going to meet somebody (who/that) I know (that) you’ll like.
It's a house (which/that) we feel (that) we might want to buy.
That's the man (who/that) I wish (that) I'd married.
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) Anna said could show us the church.
This is the woman (who/that) Anna said that could show us the church.
In this structure, people sometimes use whom as a subject pronoun. This is not generally considered correct.
This is a letter from my father, whom we hope will be fully recovered soon.
(More correct:. . . who we hope will be . . .)
Relative clauses can also be combined with if-clauses in sentences like the
I am enclosing an application form, which I should be grateful if you would sign and return.