Tricky sentences :)

How can we explain the differences in meaning within these sentences?
Jaka jest różnica w znaczeniu poniższych zdań? 

What do you do?
What are you doing?

She's been there twice.
She was there twice.

If I have a lot of money, I will buy this red car.
If I had a lot of money, I would buy this red car.

I was reading an interesting book.
I read an interesting book. 

Poproszę o odpowiedzi w komentarzach lub bezpośrednio w szkole :) 

P.Żarniewska