Relative pronouns: which vs. that

[masajac]

Test No. errors/elem-5 "Relative Pronouns", question 2

I don't understand a word what you are talking about.

(a) a
(b) what
(c) are

Test No. errors/elem-5 "Relative Pronouns", answer 2

I don't understand a word that you are talking about.

Correct entry: that
The error was: (b) what
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I don't understand why we can't use 'which' instead of 'that'. Please tell me the reason.

[Mister Micawber]

.
Which is normally reserved for non-restrictive (non-defining) clauses.

My red sneakers, which I am not wearing at the moment, are out of fashion.
.
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Native English teacher at Mister Micawber's