Question

how is simple sentences can change into compound sentence

Answer 1

the simple sentence can be changed into compound sentence by
but
yet
still
and
neither nor
either
and so
and also etc.........

Answer 2

Hyy dear by joining it with co-ordinate Claus like and, but, so, or...

A simple sentence consists of just one subject and one predicate. A compound sentence consists of two or more co-ordinate clauses, each of which has its own subject and predicate.
For example
He was guilty and went into hiding.
Hope it will help you...