I have read some grammar points about adjective clauses, but I still have problems recognizing the right choice in questions requiring them.

For example:

* *All the students ____ do well in writing.*  
Correct answer: *Dr. Freeman teaches*  
Why isn’t *"**that** Dr. Freeman teaches **them**"* correct?

And this one seems similar too:

* *The problem ____ never occurred.*  
Correct answer: *I had expected*  
Why isn’t *"**which** I had expected **it**"* correct?

Can someone please provide information or resources to get better answers?