NOUN CLAUSES 2

Grammar review Noun clauses: what vs. that

A noun clause is a dependent clause that acts as a noun, working as the subject, the object of a verb or preposition, or the complement. Noun clauses often begin with words like that, if, whether, what, who, when, where, why, and how.

The distinction this exercise tests: that and what can look interchangeable, but they play very different roles.

Use that when the clause is already complete: it has its own subject and verb, and nothing is missing:
I know that he is honest. (he is honest = complete on its own)
She said that she was tired.

Use what when something is missing from the clause: what itself fills that gap, meaning “the thing(s) that…”:
I'm interested in what he thinks. (he thinks ___; what fills the gap)
What you decide doesn't matter. (you decide ___; what is the missing object)

Quick test: try removing the word. If the rest of the clause still makes a complete sentence on its own, use that. If it doesn't (something's clearly missing), use what.