Independent clauses can stand alone as a sentence, whereas dependent clauses cannot. The sentence, Although I ate earlier, I’m still hungry, has two clauses, one independent and the other dependent. Dependent clauses generally have a subordinating conjunction or a relative pronoun in addition to their subjects and predicates and do not make sense when used as a full sentence.

An essential clause restricts the meaning of the term it modifies. The following sentence contains an essential clause (with added emphasis):

All students who do their work should pass easily.

The underlined clause adds vital information to the sentence, so it doesn’t need to be separated with commas.

A nonessential clause adds additional information, but it doesn’t restrict the meaning of the term it modifies. Although removing the clause from the sentence would cost the sentence some meaning, a reader would still understand the meaning of the term. Here’s an example:

Edgar Allen Poe, author of “The Raven,” is a great American poet.

If the underlined portion was deleted, the subject’s meaning wouldn’t change. That makes it a nonessential clause; therefore, it’s surrounded with commas.

Often, essential clauses use the word “that.” For example, “The car that I want is out of my price range.” Nonessential clauses sometimes, though not always, use the word “which.” For example, “My car, which broke down last Friday, won’t be going anywhere anytime soon.”

Coordinating conjunctions join independent clauses together. Only seven coordinating conjunctions exist: for, and, nor, but, or, yet, so. Commas are generally placed before coordinating conjunctions.

Subordinating conjunctions join dependent clauses to independent clauses. They imply that the dependent clause is less important. Many subordinating conjunctions exist: after, although, as, because, if, once, since, that, though, till, unless, until, when whenever, while, etc. Comma placement depends on how the subordinating conjunctions are used.

Correlative conjunctions are pairs of words that connect words, phrases, and clauses. Both parts of the pair have to be used in the sentence to make sense. Examples include either/or, neither/nor, both/and, whether/or, not only/but also.

Simple sentences contain one independent clause, which must simply have a subject and predicate. An example is, Bob shouted.

Compound sentences consist of two independent clauses combined with either a coordinating conjunction or punctuation like a semicolon. Here are two examples (with emphasis added):

Bob shouted a lot, but he forgave easily.

Bob shouted a lot; he also forgave easily.

See the section about coordinating conjunctions for more details.

Complex sentences have one independent clause and at least one dependent clause. Several examples are below, with their dependent clauses underlined:

Although Bob shouted a lot, he forgave easily.

She returned the book after she noticed its damage.

The movie was entertaining, just as I expected.

Experts say that money can’t buy happiness.

Compound-complex sentences have two independent clauses and a dependent clause. An example is provided below, with its dependent clause underlined and the second independent clause bolded:

Experts say that money can't buy happiness; therefore, wealthy people must not be content with their lives.

When punctuating these sentences, it's best to start with the two independent clauses since they are the largest structures. If an editor was punctuating the sentence above, he or she would first place the semicolon, then the comma after therefore. After the independent clauses is examined, the dependent clause is much easier to analyze.