Question

i know him for a simple man chnange into complex sentence

Answer 1

I know that man .He is simple

Answer 2

 Since the time that I know him, I know him to be a simple man.

This is an instance of complex sentence from a simple sentence.

A complex sentence consists of one simple sentence and in the other part it contains a noun clause, an adjective clause and an adverb clause.

So it causes of an independent clause and also another or more subordinate clauses.