Simple, compound and complex sentences
A simple sentence  has a subject and a predicate and expresses one idea.
Օրինակ
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.
Օրինակ
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.
Օրինակ
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.