The engineer who designs new software isn't here — but my boss, who is very nice, is.
A relative clause comes straight after the noun it describes. The question is whether the information is essential or extra.
I like products that use recycled materials.
My phone, which cost a lot, isn't working.
essential → no commas
extra → commas
Remove the clause: is the sentence still complete?
Essential information — it identifies the noun. No commas. who, that, which, where, when, why
Extra information — the sentence works without it. Commas always. Only who and which — never that.
My sister who lives in Rome is a doctor.
I have more than one sister. This is the Rome one.
My sister, who lives in Rome, is a doctor.
I have one sister. Rome is just extra detail.
Commas come in pairs — one before the clause, one after it.
My phone, that cost a lot, broke.
Correct: My phone, which cost a lot, broke.
The people which live upstairs are quiet.
Correct: The people who live upstairs are quiet.
The man who he works here is my cousin.
Correct: The man who works here is my cousin.
That's the village which I was born.
Correct: the village where I was born.
Say the answer out loud — then tap to check.
Lisbon, whereA I grew up thereB, isC famous forD its extraordinary light.
BI grew up there → I grew up
✓ no extra adverb after where
The novel, whichA was publishedB in 1954, wonC several prizes, and its sequels was less successfulD.
Dand its sequels was less successful → and its sequels were less successful
✓ its sequels is plural → were
My sister that lives in BerlinA isB coming forC Christmas, andD she is bringing her children.
AMy sister that lives in Berlin → My sister, who lives in Berlin,
✓ that is not used in non-defining clauses
Use a relative clause in every answer.
After a comma, never "that".