For the definition, construction, uses and general information about who, which, that, whose, whom and where, see other rules. Here is some further information:
In some defining relative clauses, it is possible to omit who, which or that.
The client (who/that) we met in London is coming to see us next week.
This is because 'the client' is the object of the verb 'met' and the subject of the verb 'is coming'. Here are some more examples:
These are the boots (which/that) I wear for hiking.
These are the people (who/whom/that) I met at the conference.
This is the report (which/that) I wrote after the conference.
The pictures (which/that) they showed us at the presentation were really interesting.
In the following, we cannot omit the relative pronoun because 'My sister' is the subject of the verb 'lives'.
My sister who lives in Paris is an architect.
My sister who lives in Paris is an architect. 
Similarly:
I'm looking forward to meeting the client who is coming next week.
I'm looking forward to meeting the client is coming next week.
Some English speakers believe that we should never end a sentence or a clause with a preposition. Instead, they place the preposition before which, whom or whose.
For example:
This is a company in which many talented people work.
It's an organisation to whose seminars people from around the world are invited.
Although this structure is acceptable in formal, written English, it sounds unnatural in conversation:
| Too formal in conversation | Preferred |
|
The friends with whom we went on holiday are coming to dinner next week.
|
The friends we went on holiday with are coming to dinner next week. |
| To whom does this bag belong?
|
Who does this bag belong to? |
|
Can you remember that Greek restaurant to which we went last year?
|
Can you remember that Greek restaurant we went to last year? |
| In whose team are you?
|
Whose team are you in? |
Do not use 'what' in relative clauses. It does not have the same meaning as 'which' and 'that'.
This is the report what I wrote after the conference. 
This is the report (which/that) I wrote after the conference. 
If there is any doubt, think of 'what' as meaning 'the thing(s) that'.
This is what I wrote after the conference. ['what' = the thing that I wrote = the report]
Can you tell me what you have done today? [the things that you have done]
I told them what they have to bring with them to the conference. [the thing or things that they have to bring]
Note that it can be used either as a subject or an object. As a subject, it can take either a singular or a plural verb:
What they showed us at the presentation was very interesting. What I liked most were the pictures.
=====All that=====
With 'all', we do not use which. We use that in this kind of sentence:
All that happened was another disagreement about the launch date - it wasn't a productive meeting.
They provided all that was required.
But in this kind of sentence, when there is a subject and verb, it's better to omit that:
All we need to do is agree on a date - then we can go home.
I'm not an expert - all I know is how to switch it on and off.
=====Of which, whom, whose=====
We can use which, whom and whose with numbers and quantifiers in this kind of sentence in order to avoid using two sentences and a pronoun:
|
They have three cars, one of which is a Renault.
|
= They have three cars. One of them is a Renault.
|
|
We stayed in a hotel in Paris, the name of which I can't remember.
|
= We stayed in a hotel in Paris. I can't remember the name of it.
|
|
There are two films on TV tonight, both of which I've seen before.
|
= There are two films on TV tonight. I've seen both of them before.
|
|
Matt has three brothers, two of whom are married.* |
= Matt has three brothers. Two of them are married.
|
|
I have lots of clients in Asia, most of whom can't speak French. *
|
= I have lots of clients in Asia. Most of them can't speak French. |
|
I deal with a company, many of whose clients are Asian.
|
= I deal with a company. Most of its clients are Asians. |
|
She's an American writer, some of whose books have been translated into French.
|
= She's an American writer. Some of her books have been translated into French. |
*Note that in these examples, who is not possible and whom doesn't sound too formal or unnatural as it does in other relative clauses.
These are a kind of noun clause. They have a 'wh' word at the beginning and are found as subjects or objects in various kinds of sentences.
Here are some examples:
1. 'wh' + infinitive is used after some verbs, including the following:
I don't know where to have the meeting or who to invite.
She informed him who to contact about the changes.
They advised us where to park.
2. When we introduce something with the verb 'to be':
This is where I'd love to live.
This is who I've invited.
3. In questions:
Do you know where to put it?
4. Sentences that imply a question or a lack of information:
I wonder who that is.
I asked her where I could find a bank.
5. As a subject:
Where we're going to launch the new product has still to be decided.
6. Note that we can also make these kinds of sentences with 'what' and 'how'. For example:
I found out what to do with these - let me explain.
She warned us what to expect if we didn't take action immediately.
He told us how to fix it.
That isn't what I meant.
Here's what I've prepared so far.
This isn't how we usually do it.
Can you remember what he said?
What we learned at the seminar was really useful.
7. When we want to be emphatic, we use a noun clause at the beginning of a sentence as a subject:
| Emphatic | Neutral |
|
What I like most about him is his honesty.
What I need right now is a strong cup of coffee and some chocolate.
What I hated about that job was the long journey every morning.
Where I'd really like to go this year is the US. |
I like his honesty most.
I need a strong cup of coffee and some chocolate.
I hated the long journey every morning in that job.
I'd really like to go to the US this year.
|
Reduced relative clauses
To make certain relative clauses more succinct, we leave out both the pronoun and the verb.
|
| Full sentence | Reduced sentence |
| With a present participle
| The people who are working on this project are the best in their field.
The people who weren't working on this project didn't have to attend the meeting.
| The people working on this project are the best in their field.
The people not working on this project didn't have to attend the meeting.
|
| With a past participle
| We ordered the equipment that was needed for the job.
We can sell any equipment that isn't needed.
| We ordered the equipment needed for the job.
We can sell any equipment not needed. |
| With some adjectives | The assistant who is responsible for booking appointments is really efficient.
Can you send a copy of the minutes to all those who were not present? | The assistant responsible for booking appointments is really efficient.
Can you send a copy of the minutes to all those not present?
|
| With a preposition | Can you look at those files that are on the desk?
Everyone who was at the conference got a free sample.
| Can you look at those files on the desk?
Everyone at the conference got a free sample. |
Note that
1. The participles remain the same whether we are talking about the past, present or future.
2. The use of present participles can represent either a simple or continuous tense. For example:
We used the laptops which belong to the IT department.
We used the laptops belonging to the IT department.
Anyone who wants to attend needs to apply before the end of the day.
Anyone wanting to attend needs to apply before the end of the day.
3. Sometimes there is either a 'simple' or 'continuous' meaning.
Passengers arriving from the EU need to queue here.
= Passengers who arrive from the EU... [all the time, regularly]
or
= Passengers who are arriving from the EU... [at the moment]
4. We can also reduce relative clauses that use the verb 'to have':
They're experts who have unique knowledge of this topic.
They're experts with unique knowledge of this topic.
It's a company which has branches nationwide.
It's a company with branches nationwide.
Denmark has the best work-life balance of the 20 OECD countries studied.
We called an urgent meeting for all those involved in the project.