I am having trouble hearing the difference between "leave" and "live." Are there any good sites that can help me hear the difference? Do you have any tips?
The two vowel sounds you are having trouble distinguishing are two that give my native Spanish-speaking students trouble, too. (leave/live, sheep/ship, feet/fit--and on and on!) There is very little difference in the position of the mouth/tongue when producing them.
You could go to http://www.soundsofenglish.org/ or http://www.howjsay.com/ and listen to them over and over.
Or, get an English speaker to say the words while you watch his/her mouth. Practice the sounds yourself while looking at yourself in a mirror. Have someone say various phrases and sentences containing the target sounds while you listen (not watching the mouth). This might sharpen your ability to hear the difference.
The greater your English vocabulary grows, the easier it will be to determine through context which words are being spoken.
Good luck! You are far from alone in this!
I made a video related to your question here ... it is a problem a lot of learners have! This video talks about tense vs. lax vowels, specifically ee vs. ih. It is part of a larger pronunciation website that I'm working on. I hope you find it useful!