Use who for people, which or that for things, and where for a place you could call there.
who, which, that, where
Ira: That's the cafe where I sketch on Sundays.
Wei: The one that has the green door?
Ira: Yes. The woman who runs it is lovely.
Not "That's the cafe which we met." — say "That's the cafe where we met.". You met there, so use where.
Try it: Amina: I need prints of my photos by Friday. Lars: Try the shop ___ I print the park maps. Choose the word for the blank. (where / which / who) Answer: where — He prints the maps there, so the place word is where.
No commas: which one?
When the added part tells you which one you mean, use no commas.
Wei: Which building are you drawing?
Ira: The one that has the blue end wall. It's on the painted row.
Try it: Lani: Which cafe do you mean? There are four on this street. Choose the reply. (The cafe that we visited on Monday. / The cafe, which we visited on Monday.) Answer: The cafe that we visited on Monday. — There are four, so the added part tells her which one and takes no commas.
Commas: extra information
When you already know who or what it is, put commas around the added part, because it is extra.
Wei: Do you know anyone at Senville Park?
Ira: Lars, who works at the visitor office, knows every path.
Not "Lars who works at the park knows every path." — say "Lars, who works at the park, knows every path.". Lars is one known person, so the extra part goes inside commas.
Try it: Mateo: Where can I study quietly on Saturday? Choose the reply. (Try Magaff Library, which opens at nine. / Try Magaff Library which opens at nine.) Answer: Try Magaff Library, which opens at nine. — There is only one Magaff Library, so the extra fact comes after a comma.
Inside commas, not that
Inside commas use who or which, not that, but without commas that works for things too.
Wei: The hall, which reopened this year, looks great now.
Ira: The choir that meets there has booked every Tuesday.
Not "The old bridge, that crosses the brook, is closed." — say "The old bridge, which crosses the brook, is closed.". With commas, use which for a thing.
Try it: Ira: Who's that by the bookings desk? Wei: That's Yasmin, ___ volunteers at the tourist office. Choose the word for the blank. (who / that / which) Answer: who — Extra information about a person after a comma takes who, not "that".
What are the common mistakes?
Incorrect
Correct
Why
That's the cafe which we met.
That's the cafe where we met.
You met there, so use where.
Lars who works at the park knows every path.
Lars, who works at the park, knows every path.
Lars is one known person, so the extra part goes inside commas.
The old bridge, that crosses the brook, is closed.
The old bridge, which crosses the brook, is closed.