Study materials
Two sentences differing only by two commas. And from them you can tell how many brothers the speaker has.
My brother who lives in Kyiv is a doctor.
My brother, who lives in Kyiv, is a doctor.
With no commas the clause narrows the field: it helps you identify which person is meant. With commas it narrows nothing and simply adds a note in the margin.
Two different jobs
One and the same structure performs two opposite functions, and commas are the only way to tell them apart in writing.
| Without commas | With commas |
|---|---|
| narrows the field, helps identify | adds a note about something already known |
| cannot be removed | can be removed, the sentence survives |
| who, which, that | only who and which |
The simplest test is to take the clause out. If the sentence still makes sense afterwards, commas are needed.
Anna, who works with me, speaks Japanese.
What goes inside the commas
Such clauses appear most often beside proper names, unique objects, and phrases like my mother or our office. That is, where there is nothing left to identify.
Kyiv, which is my home city, has changed a lot.
Our manager, who joined last year, speaks four languages.
The report, which we sent on Friday, is still unread.
That does not go in here
Here is a short rule with no exceptions. In a clause set off by commas, the word that is not used. Only who for people and which for everything else.
My brother, that lives in Kyiv, is a doctor. → My brother, who lives in Kyiv, is a doctor.
And one more thing: the pronoun cannot be dropped here. If you remember that narrowing clauses sometimes leave it out, these clauses never do.
Which about the whole sentence
A comma clause has one more possibility that a narrowing one lacks. The word which can refer not to a single noun but to the entire preceding idea.
He arrived an hour late, which annoyed everyone.
She offered to help, which was very kind.
Ukrainian usually says "and that" in such places. English always uses which here, never what.
The mistakes everyone makes
The first is using that between commas.
Kyiv, that is my home city, has changed. → Kyiv, which is my home city, has changed.
The second is putting commas where the clause narrows the field.
The man, who called yesterday, left a message. → The man who called yesterday left a message.
The third is taking what instead of which for a comment on the sentence.
He was late, what annoyed everyone. → He was late, which annoyed everyone.
All three are caught by one question. Does this clause help identify the object? If yes, no commas. If no, commas are compulsory.
Try it right now
Write two English sentences about one person you know: narrow the field with no commas in the first, and add a note inside commas in the second.
Then add a third that ends with a which comment on the whole idea.