Conjunctions are the words that join together sentences, phrases, and other words. Conjunctions are divided into several categories, each of which follows different rules.

Coordinating Conjunctions

The most common conjunctions are and, or, and but. These are all coordinating conjunctions. Coordinating conjunctions are conjunctions that join -or coordinate- two or more equivalent items (such as words, phrases, or sentences).

The acronym FANBOYS can be used to remember the most common coordinating conjunctions: for, and, nor, but, or, yet, and so. Here are some examples of them used in sentences:

• Nuclear-powered artificial hearts proved to be complicated, bulky, and expensive. (Here, the conjunction connects three items in a list, so we need commas.)
• Any external injury or internal injury puts patients at risk of uncontrolled bleeding. (Here, the conjunction connects two equivalent items but not independent clauses, so we do not use a comma.)
• The current from the storage batteries can power lights, but the current for appliances must be modified within an inverter. (Here, the conjunction connects two equivalent items -independent clauses that could stand on their own – so a comma is needed).

As you can see from the examples above, a comma only appears before these conjunctions sometimes. So how can you tell if you need a comma or not? There are three general rules to help you decide.

Rule 1: Joining Two Complete Ideas

Let’s look back at one of our example sentences:

The current from the storage batteries can power lights, but the current for appliances must be modified within an inverter.

There are two complete ideas in this sentence (a complete idea has both a subject and a verb). Because each of these ideas could stand alone as a sentence, the coordinating conjunction that joins them must be preceded by a comma. Otherwise you have a run-on sentence.

Rule 2: Joining Two Similar Items

What if there is only one complete idea but two subjects or two verbs? Consider the following examples:

1. Any external injury or internal injury puts patients at risk of uncontrolled bleeding.
2. In the 1960s, artificial heart devices did not fit well and tended to obstruct the flow of venous blood into the right atrium.

The first sentence has two subjects: external injury and internal injury. They are joined with the conjunction or. The second sentence has two verbs: did not fit well and tended to obstruct. They are joined with the conjunction and. We DO NOT use a comma with a coordinating conjunction when we are joining two subjects or two verbs, only when we are joining two complete ideas (or sentences that could stand on their own).

Rule 3: Joining Three or More Similar Items

What do you do if there are three or more items?

• Anna loves to run, David loves to hike, and Luz loves to dance.
• Fishing, hunting, and gathering were once the only ways for people to get food.
• Emanuel has a very careful schedule planned for tomorrow. He needs to work, study, exercise, eat, and clean.

As you can see in the examples above, there is a comma after each item, including the item just before the conjunction (and). There is a bit of contention about that final comma, but most styles prefer to include it (it’s called the serial comma).

Adverbial Conjunctions

Adverbial conjunctions, or conjunctive adverbs, link two separate thoughts or sentences. When used to interrupt thoughts a comma is required on either side of the conjunction.

• The first artificial hearts were made of smooth silicone rubber, which apparently caused excessive clotting and, therefore, uncontrolled bleeding.

When used to separate complete ideas (independent clauses, which each contain a subject and a verb), a semicolon is required before the conjunction and a comma after.

• The Kedeco produces 1200 watts in 17 mph winds using a 16-foot rotor; on the other hand, the Dunlite produces 2000 watts in 25 mph winds.

Be careful to think about whether the adverbial conjunction is interrupting (like therefore above) or connecting (like on the other hand).

Incorrect: My sister, therefore, is going to be the first transgender supreme court justice. (interrupting)

Correct: My sister, therefore, is going to be the first transgender supreme court justice.

Incorrect: Mexican-Americans are described by a variety of names in the United States, for example, Latinx, Chicano, and Hispanic are all terms used to describe Americans of Mexican descent. (connecting)

Correct: Mexican-Americans are described by a variety of names in the United States; for example, Latinx, Chicano, and Hispanic are all terms used to describe Americans of Mexican descent.

And remember, when connecting two complete ideas with an adverbial conjunction, use a semicolon before and a comma after.

This is a complete sentence; however, this is also a complete sentence.

Adverbial conjunctions include the following words; however, it is important to note that this is by no means a complete list.

Correlative Conjunctions

Correlative conjunctions are word pairs that work together to join words and groups of words of equal weight in a sentence.

The table below shows some examples of correlative conjunctions used in sentences:

Be careful! Correlative conjunctions can give you subject and verb agreement issues.

Correct: Neither the teacher nor the students are happy.

Incorrect: Neither the teachers nor the student are happy.

The verb m matches the closest the subject. We reviewed this in the Verbs section.

Subordinating Conjunctions

Subordinating conjunctions are conjunctions that join an independent clause and a dependent clause. Here are some examples of subordinating conjunctions:

• The heart undergoes two cardiac cycle periods: diastole, when blood enters the ventricles, and systole, when the ventricles contract and blood is pumped out of the heart.
• Whenever an electron acquires enough energy to leave its orbit, the atom is positively charged.
• She did the favor so that he would owe her one.

Let’s take a moment to look back at the previous examples. Can you see the pattern in comma usage? The commas aren’t dependent on the presence of subordinating conjunctions—they’re dependent on the placement of clauses they’re in. Let’s revisit a couple of examples and see if we can figure out the exact rules:

• The heart undergoes two cardiac cycle periods: diastole, when blood enters the ventricles, and systole, when the ventricles contract and blood is pumped out of the heart.
• The heart undergoes two cardiac cycle periods: diastole and systole. (Here we removed the extra information.)

These clauses are both extra information: information that is good to know, but not necessary for the meaning of the sentence. This means they need commas on either side (the second comma here is a period).

• Whenever an electron acquires enough energy to leave its orbit, the atom is positively charged.
  • In this sentence, the dependent clause comes before an independent clause. This means it should be followed by a comma.

Remember, the independent clause could stand on its own as a sentence.

A sentence: The atom is positively charged.

The dependent clause could NOT stand on its own.

Not a sentence: Whenever an electron acquires enough energy to leave its orbit.

• She did the favor so that he would owe her one.
  • In this sentence, the independent clause comes before a dependent clause. This means we do not use a comma.

Here’s a weird thing:

Dependent clause before independent clause – comma

So that he would owe her one, she did the favor.

Dependent clause after independent clause – no comma

• She did the favor so that he would owe her one.

The most common subordinating conjunctions in the English language are shown in the table below: