Question

I will prove to you that they are mine make it simple sentence

Answer 1

I prove they are mine.

Answer 2

"The given sentence which is a compound sentence is as follows:

I will prove to you that they are mine

On conversion to simple sentence the sentence will be as follows:

Their belongingness to me will be proved to you

Simple sentence is one where the clauses are reduced. Such sentences do not contain any conjunctions like and, but, still, only, that etc.

"