1. Don't stop him from doing what he wants. 
->  Let him do what he wants. 
2. Tom has arranged to have dinner with Ann on Friday. 
-> Tom is going to have dinner with Ann on Friday
3. We started learning English when we were in grade six. 
-> We've learnt English since we were in grade six.
4.Someone has taken some of my books away. 
-> Some of my books have been taken away
5. Tom  doesn't  smoke cigarettes any more. 
-> Tom used to smoke cigarettes