Incorrect Punctuation of Two Independent Clauses

Good writers know that correct punctuation is important to writing clear sentences. If you misuse punctuation, you risk confusing your reader and appearing careless. Notice how the placement of commas significantly affects the meaning of these sentences:

Mr. Jones, says Ms. Moore, is a boring old fool.
Mr. Jones says Ms. Moore is a boring old fool.

Writers often combine independent clauses in a single compound sentence to emphasize a close relationship between ideas. The punctuation of compound sentences varies depending upon how you connect the clauses.

Rules

1. Separate independent clauses with a comma when using a coordinating conjunction (e.g., and, but, or, for, nor, so, yet).
2. Separate independent clauses with a semicolon when not using a coordinating conjunction.
3. Separate independent clauses with a semicolon when using a conjunctive adverb (e.g., however, therefore, thus, consequently, finally, nevertheless).

Examples of Correct Punctuation, Rule A:

We all looked worse than usual, for we had stayed up studying for the exam.

This room is unbelievably hot, and I think that I am going to pass out.

Monday is a difficult day for me, so I try to prepare as much as possible on Sunday.

Examples of Correct Punctuation, Rule B:

We all looked worse than usual; we had stayed up studying for the exam.

This room is unbelievably hot; I think I am going to pass out.

Monday is a difficult day for me; I try to prepare as much as possible on Sunday. 

Examples of Correct Punctuation, Rule C:

We all looked worse than usual; nevertheless, we were relieved we had studied.

This room is unbelievably hot; therefore, I think I am going to pass out.

Monday is a difficult day for me; however, I have figured out how to prepare for it.