I have two sentences :
Jack believes himself to be a superstar
Jack thinks himself is a superstar
why is the second sentence considered ungrammatical in the light of the first sentence.
*Jack thinks himself is a superstar.
Himself is an object pronoun, an anaphor, dependent in this case on John. It can't be a subject. So it can't be the subject of himself is a superstar.
If you write 'John thinks he is a superstar', it is ambiguous. Who is he? John? Or someone else?
Obviously, the sentence 'John thinks he is a superstar.' does not stand alone in the world, but as part of a discourse. Assume, if you will, that prior to this sentence another male has been mentioned. 'Pat is a good singer.' Who does he refer to? The sentence is intrinsically ambiguous, because he does not necessarily refer to John.
Or Jack thinks himself a superstar.
You just introduced a Small Clause, a verbless clause. But it still has himself, an object pronoun as its subject, which is a bit of a contradiction.
If you put: 'Jack thinks of himself as a superstar.' that would be plain English.