microlesson:
WHO, THAT, WHICH

That is a defining relative pronoun, Which is a non-defining relative pronoun.  Who is used for people.  

Practice the subordinate clauses that start with WHO, THAT, WHICH

WHO for people

THAT for defining clauses (My bike that has a broken seat is in the garage)

WHICH for non-defining clauses (My bike, which has a broken seat, is in the garage)

I have a friend who…. 

I have a dog that… 

I have a bike that…. 

I have a job that…. 

The the project´s due date has been extended, ____ is good because otherwise we would not have finished on time.  

the weather will be nice this weekend, _____ means we wont have to cancel our plans.