Relative clauses — those subordinate clauses that add information to sentences — can be confusing, even to native speakers of English. They can help you avoid monotony in sentence structure and achieve that variety in rhythm prized in English writing.

Some relative clauses serve as giant adjectives or adverbs. For example:

You’ll find the mailboxes in the corridor where my locker is. (The where clause acts as an adjective modifying corridor.)

I didn’t check my messages last night, when I went to the theater. (The when clause acts as an adverb modifying last night, itself an adverbial phrase.)

Those are the easy ones. Where deals with place, when with time. But then there are the tricky ones:  that and which. They’re pretty much the same, right?

Well, yes; that is, they’re the relative pronouns that lead you into the clause and connect it to the rest of the sentence. And no. The difference is how they’re used.

That is used in essential clauses — those that define or limit the nouns they modify. They are, in fact, essential to the structure and meaning of the sentence; if you take them out, the sentence falls apart. It’s a little like the article the: if you’re talking about a specific (noun), use that.

Which is used in non-essential clauses — the ones that add information, such as a description, that isn’t 100 percent necessary to the sentence. Here’s an easy test: If you can lift the clause out of the sentence and it still makes sense, it’s a non-essential clause and takes which. If you can’t, it’s essential and requires that.

One more thing: which clauses are set off by commas. That never take commas.

That’s the theory. Now, some practical examples from student work I’ve seen this semester. (You know who you are.)

In 2011 the Office of the Inspector General, which controls the Department of Veterans’ Affairs, investigated the hospital after a patient died because staff members did not notice his heart monitor was disconnected.

Use which. The clause gives added information about the Office of the Inspector General that is useful to know but is not essential to the meaning of the sentence.

They claimed rapid escalation of marijuana arrests in New York City in migrants and communities of color, which are the most vulnerable populations.

Again, which is correct. You could put a period after color, and the sentence would still make sense.

One small fish can be sliced into four fillets, which are then used as bait to catch bluefish.

Same here. The sentence could end after fillets.

“Children are reinforcing their heritage,” said Gerard Lohrdal, open-space greening director of GrowNYC, a nonprofit that aims at improving New York’s quality of life through environmental programs.

The final clause defines the nonprofit, so that is correct.

Sometimes, if the meaning is clear, you don’t even have to say that; it’s understood:

In the Flatbush Food Co-op, customers can find almost everything (that) they need.

But!

“The garden can also be a bridge between the school and parents,” she explained, speaking of parents that aren’t fluent in English. 

Wrong! When referring to people, use who instead of that or which.

He has a faithful corps of students who are following his spproach: “No pain, no gain.”

Correct.

Need more practice? Click here for a quiz based on sentences from my reading in the last week.  You’ve taken the quiz and you’re still confused? Bring me your questions during office hours, or e-mail them to diane.nottle@journlaism.cuny.edu.