What is a conjunction?
Simply put, a conjunction is a short word used to connect related parts of a sentence. In the sentence below, “and” is used as a conjunction:

“I’m going to happy hour after work, and then I’m meeting Danielle for dinner.”

Conjunctions are words such as “and”, “but”, “or”, “so”, and “yet” that are commonly used to connect the main idea stated in the beginning of a sentence with a closely related idea that follows. These, along with “for” and “nor”, make up seven words known as coordinating conjunctions. (A separate article focusing on conjunctions will be forthcoming.)

What are clauses?
For our purposes here, we will be talking about both independent clauses (also known as main clauses) and dependent clauses (also known as subordinate clauses) as parts of a sentence that are joined by a conjunction. As an over-generalization, we might say that independent clauses are usually separated by a comma, which precedes the conjunction; and that an independent clause followed by a dependent clause does not use a comma. But that's an oversimplification.

Independent clauses
If both parts (clauses) of the sentence could stand alone as complete sentences, each containing a subject and a verb, then these are independent clauses. The conjunction joining them normally would be preceded by a comma. (This definition assumes that it is not a run-on sentence that should be split into two separate sentences.)

Example: “I’m going to meet Pam at the corner, and we are going to go to the café for lunch.”

Explanation: Each of these clauses could stand alone as a complete sentence with a subject and predicate. We don’t make them two separate sentences, because they’re conveying closely related thoughts, and separating them would sound choppy and stilted.

“I’m going to meet Pam at the corner.”
“We are going to go to the café for lunch.”

This rule includes the case in which the subject of the clause may not be explicitly stated but is implied, such as in an imperative statement in which “you” is understood but not stated.

“Meet Pam in the theater lobby, and take her to the café for lunch.”

Exception: Short, closely-related clauses do not require the comma.

“Pam ate lasagna and I had calzone.”

Exception to the exception: A list of short clauses does use commas.

“Pam ate lasagna, Peter ordered pizza, I had calzone, and Jerry wolfed down a whole cherry pie."

Dependent clauses:
A dependent clause could not stand alone as a sentence and have the same meaning, and no comma is used before the conjunction.

“I’m going to change into shorts after school and run around the block.”
“Peter said that he will come over today and bring lunch.”

No comma is used with a restrictive dependent clause that follows a main clause. Examples of restrictive words are “if” and “when”. (These belong to a group of words called subordinating conjunctions.)

“Let’s go to a movie this afternoon if it rains.”

However, a comma is used if the restrictive dependent clause precedes the main clause:

“If it rains this afternoon, let’s go to a movie.”

A dependent clause that is supplementary or parenthetical (adds information without changing the meaning) should be preceded by a comma.

"Peter went fishing, for reasons known only to himself, in spite of the afternoon rainstorm."
"Peter kept fishing through the downpour, for reasons known only to himself."

In some cases, use of the comma determines the meaning of the sentence:

“I didn’t leap across the raging creek because I was fearful.”

The above sentence, with no comma, means: I leaped (or leapt) across the raging creek, but not because I was fearful.

“I didn’t leap across the raging creek, because I was fearful.”

With the comma, the sentence means that I did not leap across the raging creek, and the reason I didn’t is that I was fearful.

It could be restated more clearly:

“Because I was fearful, I didn’t leap across the raging creek.

Reference materials:
1. Chicago Manual of Style, 16th edition.
2. Website “Daily Writing Tips”. https://www.dailywritingtips.com/8-types-of-parenthetical-phrases/. Article: “8 Types of Parenthetical Phrases” by Mark Nichol.

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