How do you change simple sentences to compound sentences?

You can combine two or more simple sentences by using conjuctions. Except for very short sentences, you should put a comma before some conjuctions, which are called coordinating conjunctions.

They are :

for

and

nor

but

or

yet

so

(the mnemonic is fanboys).

The subordinating conjunctions allow one independent clause to modify another.

They include :

after

although

as

as if

as long as

as though

because

before

even if

even though

if

if only

in order that

now that

once

rather than

since

so that

than

that

though

till

unless

until

when

whenever

where

whereas

wherever

while