Even though punctuation can sometimes be confusing, the information below should help answer some of your punctuation questions.
The period is the proper end mark for an independent clause, which is a complete thought consisting of a subject and a verb.
The semicolon is used to join two independent clauses that are logically connected. If a comma is used to connect the sentences, the result is a common sentence error called a comma splice. Commas don’t join sentences.
Words like therefore, however, nevertheless, consequently, furthermore, and moreover are called transitional expressions or independent markers and are set off by commas. If they are used to join two independent clauses, put a semicolon before the transitional expression and a comma after the transitional expression.
And, but, yet, so, or, nor, and for are coordinating conjunctions and are used to join independent clauses. This juncture is often signaled by a comma.
Another use of commas is to signal and set off introductory words, phrases, or clauses and nonessential clauses or parenthetical expressions.
The clauses that subordinating conjunctions (if, because, since, when, while, although, after. . . ) introduce are dependent and cannot be punctuated as complete sentences. Remember that although these clauses have subjects and verbs, the subordinating conjunction makes it dependent and unable to stand alone. If the sentence begins with a dependent clause, a comma goes at the end of the clause (and right before the beginning of the next clause).
Contrary to the comma rule just stated, when a dependent clause, like a subordinate clause, is located at the end of the sentence, no comma is needed.
Colons are used to set up lists, whether that list is comprised of words, phrases, or short clauses. The items in the list are separated by commas.
Commas are also used to set off direct quotations. Commas and all punctuation must be inside the quotation marks.
Source: Behrens, Laurence and Leonard J. Rosen. Writing and Reading Across the Curriculum. 4th ed. 1991.