1. I’ve never heard of the people and things. You talked about them just now.

I’ve never heard of the people and things which you talked about just now.  

2. You are asking a question. The question is the only one that hasn’t been answered.

The question you are asking is the only one that hasn’t been answered.  

3. We visited the museum last Sunday. The museum is being decorated now.

The museum (that) we visited last Sunday is being decorated now.  

4. That evening I ended up working very late. I will tell you more about it later.

That evening, which I will tell you more about later, I ended up working very late.  

5. I wish to thank Professor Smith. Without Smith’s help I would never have got this opportunity.

I wish to thank Professor Smith, without whose help I would never have got this opportunity.