1, This is the best computer I have ever used
-> I have never used such a good computer.
2, He hasn't chatted with his classmates throught internet for five years
-> It was five years ago since he last chatted with classmates throught internet,
3, He has never played a computer game before 
-> This is the first time he has ever played a computer
4, We haven't seen her since we left university 
-> The last time I met her was when we left university.
5, He last comeback to his homevillage for five months ago
-> He hasn't comeback to his homevillage for five months
6, It's three years since I last spoke to her
-> I haven't spoken to her for three years.