A common problem writers face is whether to use a comma between two clauses. The solution depends on whether one the clauses is essential or non-essential. Following are some useful guidelines.
Let’s start by exploring the difference between essential and non-essential clauses.
An essential clause contains words that modify the main thought of a sentence in a way that’s important to the main thought. Without the essential clause, the main thought would be unclear or incomplete. An essential clause is never set off with a comma or commas. Essential clauses usually begin with a subordinating conjunction (see below). Examples:
John stayed home from school because he was sick and contagious. Being “sick and contagious” is essential to the reason John stayed home, as opposed to any other possible reason for John staying home.
The woman dressed all in black was either a widow or an interesting dresser. The information about the woman would not likely be true of any other woman in view, so it’s essential.
A non-essential clause contains information that’s not necessary to complete the main thought. Commas are inserted in order to separate it from the rest of the sentence. Non-essential clauses also usually begin with a subordinating conjunction (see below). Examples:
John stayed home from school today, even though he wasn’t really sick. The main thought “John stayed home from school today” would be true, without any further explanation. It’s a non-essential clause, so it gets a comma inserted.
Jane, who dyes her hair, is my best friend. Jane is my best friend, regardless of whether she dyes her hair or not. So that clause is non-essential and is set off by commas.
A subordinating conjunction is a connecting word or phrase that joins a subordinate clause to a main clause. The subordinate clause is used to redefine or modify the main point of the sentence. Subordinate clauses may come first in a sentence, but they’re still subordinate. Why? Without the main point, they can’t stand alone.
Most subordinating conjunctions consist of a single word (see list below). However, some of them may contain more than one word (again, see list below). There are several subordinating conjunction categories, based on the kind of meaning they express.
- Subordinating conjunctions about cause—These explain why an activity in the main clause happened. Words include: as, because, in order that, since, so that. Example: I like to hiking and sightseeing because they’re stimulating.
- Subordinating conjunctions about condition—These contain information about the circumstances under which the main clause will be performed. Words include: even if, if, in case, in the event (that), just in case, only if, provided (that), unless, whether. Example: Even if it rains on the weekend, we’ll still go camping.
- Subordinating conjunctions about time—These determine a moment or interval when the main clause will be performed. Words include: after, as soon as, as long as, before, by the time, every time, now that, once, still, till (or ’til), until, when, whenever, while. Example: He re-hid his little “treasure” after everyone had left.
- Subordinating conjunctions about place—These determine location of activities in the main clause. Words include: where, whereas, wherever. Example: He hid his little “treasure” where he was sure that no one would find it.
- Subordinating conjunctions about contrast—These modify the main clause in the context of the process being discussed. Words include: although, as if, as though, even though, in contrast to, just as, though, whereas, whether or not, while. Examples: Henry went to the movies, even though he’d been told not to. John never misbehaved, whereas his sister Jane was very mischievous.
Some other common subordinating conjunctions include: how, inasmuch, than, that, who.
If you’re a writer, you need to know about essential and non-essential clauses and about subordinate conjunctions. This includes when and how to use commas. Without this knowledge, your meaning of a sentence may change or not be understood by readers.
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