Other Function of that
A problem with identifying English relative clauses is that the same words that serve as relative clause introducers have other functions. The word that is used as a “demonstrative pronoun” as in that book or I find that interesting. The word that is also used to introduce “that-clauses” (sometimes called “noun clauses”) as in the sentence below.
The photographer said that the image should be enlarged.
Wh-Words/Phrases in Other Embedded Structures
In addition to introducing relative clauses, wh-words and phrases are used to introduce various kinds of embedded sentence structures as in the following sentences:
The professor asked who was interested in digital photography.
The students were arguing about whose camera should be used for the project.
Unlike relative clauses, that-clauses and these embedded wh-clauses occur in positions that regular noun phrase objects can occur in. Compare the above three sentences with the following three sentences:
The photographer said something.
The professor asked a question.
The students were arguing about a decision.
A relative clause, on the other hand, occurs inside a noun phrase and modifies the noun that it immediately follows.
Of course, another function of wh-words and phrases is that they are used in “wh-questions” as in the following examples:
Who did you buy that scanner from?
From whom did you buy a scanner?
Which scanner did you buy?
Whose scanner did you buy?