1.He plays the guitar better than I do.
=>I don't         play the guitar as well as he does
2.How long have you been learning English?
=>When    have you started to learn E.?
3.She didn't realize it was valuable. She gave it away.
=>If       she didn't realize it was valuable, she gave it away
4.Unfortunately I've got to work late tonight.
=>I wish       i get to work early tonight
5.Don't forget to contact me if you come to Vietnam.(keep)
=>Don't forget    to keep calling when you come to VN