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Explanation
A simple sentence  has a subject and a predicate and expresses one idea.
I like apple pie.
Nare and Vardan waited for the train.
A compound sentence  is two simple sentences joined by a comma and a conjunction such as and, but, etc.
A compound sentence expresses  two complete thoughts.
Nare and Vardan waited for the train, but the train was late.
Nare and Vardan arrived at the station early, and they left before I arrived.

A complex sentence  is made up of a main clause and one or more dependent clauses.
The dependent clause usually begins with a subordinating conjunction such as  because, as, etc.
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Because Nare and Vardan arrived at the station early, I did not see them at the station.
I did not see them at the station because Nare and Vardan arrived at the station early.
As-clauses  often come at the beginning of sentences. As is used when the reason is already known.
Because-clauses  usually comes at the end.Because  puts more emphasis on the reason.
As he wasn’t ready, we went without him.
We went without him because/as he wasn’t ready.