A simple sentence has one subject and one predicate.
Mary plays tennis.
A simple sentence with a compound subject and compound predicate:
Mary and Tom play tennis and swim.
A compound sentence has more than one part that can stand alone (independent clauses). Independent clauses are connected by coordinating conjunctions, conjunctive adverbs or a semi-colon.
Tom swims, and Mary plays tennis.
FOR, AND, NOR, BUT, OR, YET, SO
MOREOVER, HOWEVER, OTHERWISE, THEREFORE
Conjunctive adverbs are sometimes called “floating” adverbs because they can be positioned at the beginning, in the middle, or at the end of a clause.
Bob is handsome; moreover, he is rich.
Bob is handsome; he is, moreover, rich.Bob is handsome; he is rich, moreover.
“If the relation between the ideas expressed in the main clauses is very close and obvious without a conjunction, you can separate the clauses with a semicolon” (Little, Brown Handbook, 9th Edition, p. 361).
A complex sentence has at least two parts: one that can stand alone and another one that cannot. The part that cannot stand alone is linked to the rest of the sentence by a subordinating conjunction. The most common subordinating conjunctions are "after," "although," "as," "because," "before," "how," "if," "once," "since," "than," "that," though," "till," "until," "when," "where," "whether,” and “while."
Bob is popular because he is rich.
Since Bob is rich, he is popular.
This type of sentence has more than one part that can stand alone, and at least one that cannot. Conjunctions link the different parts of this sentence.
Bob is popular because he is rich, but he is not very happy.