Learn English – Difference between that and what

cleft-constructionsrelative-pronouns

I'm struggling to find the difference between that and what especially when using in the following sentence.
The sentence that makes me confused is:

It is concatenation that does the trick.

In the sentence above, is "that" used as a relative pronoun? Or is the whole sentence "It is..that" cleft sentence?
And why can't I use "what" instead of "that?"
Can't I write it like:

It is concatenation what does the trick.
(=What does the trick is concatenation.)

Is this grammatically wrong? If so, could you let me know the reason?

I really need your help.

Best Answer

As you said,

It is concatenation that does the trick.

is a cleft sentence.

What does the trick is concatenation.

is also a cleft sentence which can also be expressed this way:

Concatenation is what does the trick.

However,

It is concatenation what does the trick.

is grammatically wrong because in it-cleft sentences what is not used. Instead, we use that or who when we refer to a person.

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