Ok, I told you yesterday that there was one more use of commas I’d save till today. Have a look at the sentences below, and tell me which of the two people quoted has only one brother:

My brother who works in the bank visited today.

My brother, who works in the bank, visited today.

If you said the second person only has one brother, you’re correct! But how did you know?

The answer of course, lies in the commas. In the first sentence, the information …who works in the bank is essential. This person has multiple brothers, so it’s essential that they include the information which tells us which one they’re talking about. And because it’s essential, we don’t put commas around it.

But the second person has only one brother, and the information …who works in a bank is therefore not essential to the sentence. We know who they’re talking about without it. Because it’s not essential, we separate it from the main body of the sentence by wrapping it in commas. Let’s look at two more quick examples:

The man who lives next door has won the lottery!

Tom Walsh, whom I was talking to just five minutes ago, has won the lottery!

In the second sentence, we obviously who Tom Walsh is, and the fact that the speaker’s just spoken to him isn’t necessary for us to understand that he’s won the lottery. It’s just a nice bit of extra information. But in the first sentence we can’t say The man has won the lottery! The additional information about this man is essential for us to identify him.

When we add extra information like this about someone or something, it’s called a relative clause. When it’s essential to the meaning of the sentence it’s a defining relative clause, and if it’s not essential, we call it a non-defining relative clause. We native speakers usually don’t have trouble with them. We might sometimes add commas when we shouldn’t, or vice versa. And Donald Trump doesn’t really use them at all. Which isn’t surprising, as learners of English often avoid using complex sentences like these, and given Donald’s level of English, it’s understandable that he does the same.

I only really mention relative clauses because they make me feel nostalgic. Back in the long-ago days of 2008, I was applying for the training course to teach English (basically the CELTA, if you’re familiar with the industry). As part of the application, we had to answer a few language questions. One was basically the question that I began with. I still remember how proud I was to have figured out the difference between the two, though I did find it hard to express the difference succinctly. Which I now realise they were probably testing me on too, because that’s obviously important in teaching.

There were some other teaching-focused questions too. Like, How would you teach the word handbag to beginners? Clever clogs me said if there were a handbag in the classroom I’d indicate it, which was totally the right answer. I’m still haunted by the one language question I didn’t get right though. Asked to identify the conjunction in a sentence, I highlighted the whole clause featuring the conjunction (which I think was but). I wasn’t confident I was right, and when I checked and got home, I felt like kicking myself. Still, in retrospect, it wasn’t a bad attempt, and I’m pretty sure I got everything else right.

Which was reassuring. I knew at the time that I was a grammar nerd, but I had no idea if that could translate into me being a good language teacher. And even after passing the course I still had a lot to learn. But doing that little test reassured me that I was on the right course. And I’m so glad I chose that course, because beforehand I’d never really considered language teaching as something I might be really into. I never really planned to do it for long, but here I am! And it’s sobering to think that without that little test that cold January afternoon nine years ago, I probably wouldn’t be here writing to you now. Which is why I always enjoy teaching relative clauses, and getting students to notice the difference. Though I prefer to go with My wife (,) who’s from France (,) is a good cook: much more memorable!

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