Relative clauses

What are the typical mistakes we make with these sentences? Can you spend two minutes testing yourself? By the way do you use ‘who his / who her’ instead of ‘whose’?

As you probably remember there are defining and non-defining relative clauses. Can you spend just a minute deciding? Relative clauses – defining or non-defining?

The also offers a great explanation of relative pronouns. 

An explanation in youtube:

A good page with explanations and exercises from EOI Langreo.

But there is more: exercises 1exercises  2.