Look at these examples to see how defining relative clauses are used.
Are you the one who sent me the email?
The phone which has the most features is also the most expensive.
This is the video that I wanted to show you.
The person they spoke to was really helpful.
Try this exercise to test your grammar.
Read the explanation to learn more.
Relative clauses give us information about the person or thing mentioned.
Defining relative clauses give us essential information – information that tells us who or what we are talking about.
The woman who lives next door works in a bank.
These are the flights that have been cancelled.
We usually use a relative pronoun or adverb to start a defining relative clause: who, which, that, when, where or whose.
We can use who or that to talk about people. that is more common and a bit more informal.
She's the woman who cuts my hair.
He's the man that I met at the conference.
We can use which or that to talk about things. that is more common and a bit more informal.
There was a one-year guarantee which came with the TV.
The laptop that I bought last week has started making a strange noise!
when can refer to a time.
Summer is the season when I'm happiest.
where can refer to a place.
That's the stadium where Real Madrid play.
whose refers to the person that something belongs to.
He's a musician whose albums have sold millions.
Omitting the relative pronoun
Sometimes we can leave out the relative pronoun. For example, we can usually leave out who, which or that if it is followed by a subject.
The assistant [that] we met was really kind.
(we = subject, can omit that)
We can't usually leave it out if it is followed by a verb.
The assistant that helped us was really kind.
(helped = verb, can't omit that)
Do this exercise to test your grammar again.
I'm not sure why a language works in the way it does is a question that has an answer. Languages evolve and develop their rules and systems over time; there is no planned purpose which would imply a reason for a particular rule. It is as it is, simply.
Note that we can use that to replace the relative pronouns which and who, not whose and not the relative adverbs where and when. We also do not use that in non-defining relative clauses.
I'm afraid we don't offer a proofreading or correction service on LearnEnglish. We're a small team and it's just not possible for us to do this. We focus on explaining difficult areas of English and offering advice to learners.
The sentence you quote in your third question is incorrect. You need to include the auxiliary verb be when forming the present continuous and not only use the -ing form:
The LearnEnglish Team
Yes, both which and that are possible here. It's also possible to omit the relative pronoun entirely.
The LearnEnglish Team