Let's move on to clauses. Clauses are a bit more complicated, as they come in several types. But all of them have a subject and a verb. Take our earlier examples:

She rode her bike to class.

When she rode her bike to class.

The first example is a sentence, while the second is not. But both are clauses. The difference is that one is an independent clause and the other is a subordinate clause.

An independent clause can stand on its own as a sentence. A subordinate clause, in contrast, starts with a subordinating conjunction or a relative pronoun (such as who, which, or what) and cannot stand on its own. Instead, it must be connected to an independent clause.

Here is an example of an independent clause:

They ran the marathon.

Now, let's transform it into a subordinate clause by (1) replacing They with a relative pronoun (who), and (2) attaching it to an independent clause.

The crowd cheered the athletes who ran the marathon.

In this sentence, the subordinate clause is restrictive — that is, it supplies essential information, telling us which athletes (out of all the athletes in the world) received the applause. When a subordinate clause is restrictive, it isn't set off with a comma.

Not surprisingly, some subordinate clauses are nonrestrictive, meaning that the information they supply is not essential. For example:

My sock has a hole, which resulted from my walking on a cement floor.

Here, the clause which resulted from my walking on a cement floor adds detail, but we don't need it to figure out whose sock the writer is talking about. So the clause is nonrestrictive, and the writer properly set it off with a comma. Same deal here:

I put a twist of lemon, which is a tasty fruit, in my drink.

Here, the nonrestrictive clause comes in the middle of the sentence, so it needs commas on both sides.

A rule of thumb: Clauses starting with which are almost always nonrestrictive.

I have said that words such as who, which, and what can serve as relative pronouns introducing a subordinate clause. In a question, however, they can operate differently. Consider this example:

Who ran the marathon?

Here, who is the subject of the question — a stand-in for the unknown identity of the marathon runner. And such a question qualifies as an independent clause.

Analyzing Sentences

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