Do you know the difference between them?
If you still mistake them, you will probably find interesting the following link:
Don't worry if you don't understand everything the first time, the speaker speaks really fast.
Look the video, and listen to it twice, it's sure the second time you are going to understand a bit more.
Anyway, it's a good way to practise the listenning for the exams, moreover if you are studying at the second year of advanced level.
If your english level is lower, try one of the following links:
And, if you have any doubt after watching them, then read the explanation in the following video:http://www.youtube.com/watch?v=djkZcaIpBH4