You Have Three Choices in Punctuating Compound Sentences
Whenever two or more independent clauses (i.e., phrases that could stand
alone as sentences) are joined together, there are always three punctuation
options.
1. Split them into separate sentences.
2. Use a semicolon to separate the clauses.
3. Use a comma before the conjunction that connects the clauses.
Consider, for example, the following sentence. Notice how it can be
punctuated—properly—three ways. There is no right or best choice
among the three; they’re all acceptable alternatives. As you read them,
ask yourself which one you like best and why.
1. Mindy wanted to apply for the newly created position. Tom did not.
2. Mindy wanted to apply for the newly created position; Tom did not.
3. Mindy wanted to apply for the newly created position, but Tom did not.
The first option, separating the two clauses, results in a short second
sentence that serves to emphasize Tom’s stance. The second option
employs a semicolon to show that the sentences are intimately connected.
The third option adds a conjunction to show the relationship between the
two sentences.
Choose the one that seems best to you and that most reflects your
meaning.
Separate a Dependent Clause from an Independent Clause
A dependent clause at the beginning of a sentence should be separated
from the independent clause that follows it by a comma. (Dependent
clauses often start with the words after, although, as, because, if, since,
when, or while, among other words.)
• Although Mr. Samuels meant it for the best, his comment was
counterproductive.
• As it turned out, we were able to attend the conference.
• Because of the change in leadership, the project deadline was
extended.