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.