The four possible structures of a sentence:
- A simple sentence, which contains an independent clause
- A compound sentence, which contains two or more independent clauses connected with a coordinating conjunction or a semicolon
- A complex sentence, which contains one independent clause and one or more dependent clauses
- A compound-complex sentence, which is a hybrid. It is a variation on a compound sentence where a complex sentence has replaced one or both of the independent clauses.
Here are some examples:
A simple sentence contains just one independent clause. However, “simple” does not automatically mean short. There are six prepositional phrases in the example below, but it remains a simple sentence because there are no other clauses:
They came at last to a broad estuary flowing eastward through the barrier beaches with leaping tongues of surf on either side where it met the sea.¹
While the prepositional phrases in the example above did not need commas, simple sentences may contain phrases that are separated with commas and still be “simple.”
Edinburgh, the capital city of Scotland, is the permanent host of the International Festival Fringe.
In the case of the example above, the underlined phrase is a redefinition of “Edinburgh.” It needs commas to indicate it is an interruption of the sentence. Commas are also used with names (e.g., “Daniel’s brother, Samuel, married my sister, Jane.”), places (e.g., “They honeymooned in Paris, Texas, instead of Paris, France.”), dates (e.g., “The wedding took place on October 31, 2002, in the church cemetery at dusk.”), and transitions (e.g., “Since then, Halloween parties have been their personal Valentine’s Day.”).
[If you noticed the “e.g.” and aren’t sure why I used it instead of “i.e.” then click here.]
A compound sentence is made by connecting two simple sentences. There are three ways to make the connection. One way links them by one of seven possible coordinating conjunctions — perhaps in your past you used FANBOYS as a mnemonic: for, and, nor, but, or, yet, so.
The wind died, but the air seemed colder.
Notice that the comma comes before the coordinating conjunction, not after.
Another way to create a compound sentence uses a semicolon:
The wind died; however, the air seemed colder.
Conjunctive adverbs, such as however, therefore, finally, and furthermore, are used immediately after a semicolon joins two independent clauses. They are considered weaker links than the FANBOYS coordinating conjunctions. (Please note: the semicolon is not used if what comes before the conjunctive adverb is a phrase and not an independent clause.)
The final way lets a semicolon replace a coordinating conjunction when a writer wants the two independent clauses to be very closely associated and without the slight shift in thinking that happens with one of the FANBOYS. Like this:
The wind died; the air seemed colder.
Like simple sentences, complex sentences have one independent clause. Unlike simple (or compound) sentences, however, complex sentences also contain one or more dependent clauses. Dependent clauses begin with a subordinating conjunction, which indicate a relationship between the two clauses. The most commonly used are although, because, when, while, and until (but there are many more than just those). In this example, the dependent clause interrupts the independent clause so commas are used to set it apart:
The dog, just a stray which had been found half starving beside a road some years ago, went everywhere the old man went.¹
Dependent clauses add information to the independent clause, but can be removed from a sentence without losing the central meaning. The reverse is not true: reading the dependent clause alone is not a complete thought.
Like a compound sentence, a compound-complex has two parts joined by either one of the FANBOYS conjunctions or a semicolon. The difference is that one or both parts is a complex sentence (independent plus one or more dependents) rather than two independent clauses.
The wind died, but the air seemed colder because the sun had now set.
The wind died, but because the sun had now set, the air seemed colder.
The above examples are variations of the one I used for compound sentences; however, to make one of the parts complex, I added a dependent clause (underlined). When the dependent clause follows the independent clause and then the entire sentence ends, a comma is not used. When a dependent clause comes before the independent clause, use a comma to separate it.
Few people have trouble with short lists, “bread and cheese” or “wine or beer.” Sometimes though, when the items are wordy, people pop a comma between the two out of habit. Do not use a comma when the “and” or “or” link phrases and not clauses. For example, this “and” links phrases: I bought day-old Scottish baps (potato rolls) and a small jug of single malt whiskey for a picnic.
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Many of us had teachers who, in trying to be helpful, told us commas land at the point(s) where someone needs to take a breath. Speech probably does pause slightly in the same places written words get commas, but not so speakers or readers can take a breath and not every time. It is more likely that conversation pauses are similar to what is happening on a page: the pause (the comma) is a signal that there is a shift happening from, for example, a partial thought (dependent clause or interrupting phrase) towards a main thought (independent clause).
¹ Gordon, Arthur. “Not Far From Sundown.” The Best of Field & Stream, 100 Years of Great Writing. Ed. J. I. Merritt. New York: Lyons & Burford, 1995. 57-63. Print.