To add information about a person, join the two sentences with who. The who takes the place of he, she, or they.
who joins two sentences about a person
The man is a doctor. He lives next door. → The man who lives next door is a doctor.
I have a friend. She speaks five languages. → I have a friend who speaks five languages.
Not "The man which lives next door is a doctor." — say "The man who lives next door is a doctor.". For a person, use who, not which.
Try it: I have a friend. She speaks five languages. Join the two sentences into one. (Leave out the word you don't need.) Answer: I have a friend who speaks five languages. — A friend is a person, so join with who: I have a friend who speaks five languages. "Which" is for things, so leave it out.
which or that joins two sentences about a thing
To add information about a thing or an animal, join with which or that. Both are fine.
This is the bus. It goes downtown. → This is the bus that goes downtown.
I like films. They have happy endings. → I like films which have happy endings.
Not "I like movies who have happy endings." — say "I like movies that have happy endings.". Movies are things, so use that or which, not who.
Try it: This is the bus. It goes downtown. Join the two sentences into one. Answer: This is the bus that goes downtown. — A bus is a thing, so join with that (or which): This is the bus that goes downtown. "Who" is only for people, so leave it out.
You can leave out who / which / that
When who, which, or that is the receiver — someone else does the action to it — you can leave it out: "the film that I saw" = "the film I saw". Keep it when it is the doer, the one doing the action.
This is the film. I saw it last night. → This is the film (that) I saw last night.
Hiroshi is the friend. Mei met him. → Hiroshi is the friend (who) Mei met.
Here is the book. You lent it to me. → Here is the book (that) you lent me.
Not "This is the song I like it." — say "This is the song I like.". When you drop the joining word, don't add it back: the song I like, not "the song I like it".
Try it: This is the book. Amina recommended it. Join the two sentences, leaving out the joining word. (Leave out the word you don't need.) Answer: This is the book Amina recommended. — Amina does the action, so the book is the receiver — you can drop the joining word: This is the book Amina recommended. You do not need "that", and you must not add "it" back after recommended, so leave both out.
where joins two sentences about a place
To add information about a place, join with where.
That's the restaurant. We ate there. → That's the restaurant where we ate.
This is the place. We met here. → This is the place where we met.
Not "This is the place which we met." — say "This is the place where we met.". For a place where something happens, use where, not which.
Try it: This is the cafe. We first met here. Join the two sentences into one. (Leave out the word you don't need.) Answer: This is the cafe where we first met. — A cafe is a place you do something in, so join with where: This is the cafe where we first met. "Who" is for people, so leave it out.
Try it: Amina is the teacher. She helped me. Join the two sentences. Pick the right joining word and leave the others out. Answer: Amina is the teacher who helped me. — A teacher is a person, so the joining word is who: Amina is the teacher who helped me. "Which" is for things and "where" is for places, so leave both out.