Study materials
A single punctuation mark can change how many brothers the speaker has. That is not a joke but the ordinary work of commas in a complex sentence.
The students who passed the exam went home.
The students, who passed the exam, went home.
The commas show whether the clause is needed for identification. Every complex relative sentence starts from that decision.
Commas decide the meaning
This pair of sentences differs only in punctuation, and they report different things.
My brother who lives in Kyiv is a doctor.
My brother, who lives in Kyiv, is a doctor.
Without commas the clause narrows the field and is needed for identification. With commas it narrows nothing, and the sentence still makes sense without it.
Two practical consequences follow. A comma clause never uses that, and its pronoun can never be dropped.
What can be dropped
In a clause with no commas the pronoun is sometimes left out. The rule is simple: if a subject of its own follows immediately, the pronoun can go.
the film that we watched → the film we watched
the man who lives next door
A comma clause offers no such choice: there the pronoun always stays.
Which about a whole sentence
A comma clause can do one more thing a narrowing clause cannot. The word which may refer not to a noun but to the entire preceding idea.
He arrived an hour late, which annoyed everyone.
Ukrainian says "and that" here. English always uses which, 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.
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?
Try it right now
Write two English sentences about people at your work: narrow the field with no commas in one, add a detail inside commas in the other.
Then add a third that ends with a which comment on the whole idea.