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1. I like to go to the club whose? owner is one of my old friends.

I like to go to the club where the owner is one of my old friends.

Which one is correct ? Why?

2. Why do you still want to work at the company who? treats you unfairly? Is it correct?

3. I know some places which sell good kitchenware.

I know some places where they sell good kitchenware. Which is correct? Why?

4. I think I will stay at/on? the job because I'm tired of looking for jobs.

What's the difference between them?

Thanks for being so patient answering my questions.
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1) and 2) who/whose is used about people or possibly some animals. You can't use it for organisations.

3. both options are correct. There is often more than one way to express something.

4. AT this job. (on the job is a euphamism for something you really don't mean Emotion: smile)

What's important is that you have to stick with your plan. It's important that you stick with your plan.

Both are correct so either can be used but they have slightly different meanings. 'It's important' is just a simple statement.- a straightforward instruction. 'What's important' suggests that there are other things to be considered but the most important thing at the moment is that you have to stick with your plan.
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Sorry, I missed one question

What's important is that you have to stick with your plan. Is it correct to say so?

Compared with It's important that you stick with your plan, which one is more commonly used?
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 nona the brit's reply was promoted to an answer.
So in terms of (= according to?) your explanation, for 1 I should use "I like to go to the club where the owner is one of my old friends." instead of "I like to go to the club whose owner is one of my old friend?"

For 2, isn't it kind of weird to say, "you still want to work at the company which treats you unfairly?" I found sentences in some articles( Would it be more appropriate to put "in some articles" at the end?) in which the authors use who instead of which. Is it commonly used for conversation?

Thanks a lot for answering my questions!!
Nona The Brit 1) and 2) who/whose is used about people or possibly some animals. You can't use it for organisations.
This is what I thought until some time ago.

Then I read:
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whose
A. Meaning "of which"


Whose may usefully refer to things <an idea whose time has come>. This use of whose, formerly decried by some 19-th century grammarians and their predecessors, is often an inescapable way of avoiding clumsiness--e.g.:

--A book whose humour could have seemed tediously affected turns out to be entertaining and utterly tasteful. Atlanta J. & Const. 1995.

--About $4.1 million is from buildings whose owners filed for bankruptcy, she said in an interview. Washington Post, 1998.

Garner, Modern American Usage, p. 537
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