Can someone help me understand the grammar behind the following sentence?
It’s not what you think it is.
I completely understand the meaning of it. For some reason I thought sentence would be written as:
It's not what you think what it is.
Just like when you say "It is what it is".
"what you think it is" is a content clause with the interrogative "what". To understand it, you should compare it to an independent clause:
you think it is (a dog)
There is only one "gap" here. Saying "you think what it is a dog" is incorrect, and "you think what it is is a dog" is correct, but unnecessary. The verb think can be completed by a clause like "it is a dog".
So when forming the content clause, the noun phrase is omitted and an interrogative pronoun is placed at the start of the clause: "what you think it is". No second what is required.
As suggested above "It's not what you think what it is, is" could be considered "correct", but only if you are trying to make a joke about English grammar!
The second 'what' is redundant.
'What' is being used the same way in both of the sentences you provided. The meaning of 'what' in these instances is: the identity of an unknown thing. Let me rephrase each part of the sentence into other words.
It's not what you think it is
The words inside of parenthesis are what the words outside of parenthesis mean.
It's not (Not exist) what (The identity) you think (you think that) it is(exists).
If you were to add another 'what' it wouldn't make sense, because it is sufficient as is.
It's similar to why we add pronouns instead of saying a person's name repeatedly throughout a sentence; this is to avoid redundancy.
