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.

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 )

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.

Nona The Brit wrote:

1) and 2) who/whose is used about people or possibly some animals. You can't use it for organisations.