Explanations

This verb is rather special in phonetics because it may be pronounced in different ways, and it is very important to make the difference, or nobody will understand if you say it in the affirmative or in the negative, so here are some explanations and videos.

CAN / CAN'T

The most important thing is to know that the difference between CAN and CAN'T is not the T, because the T usually disappears in a normal conversation. The most important thing to know if it's affirmative or negative is not the T, it is the vowel:

CAN Usually, we don't pronounce the vowel, so it sounds /kn/

CAN'T Usually, we don't pronounce the final T, but we pronounce the vowel in the middle strong and clear. Now, about this vowel, there's a difference between standard British accent and American accent:
Standard British English pronounces this with the vowel in CAR /kɑ:n/
American English (and some non-standard varieties of BrE) pronounce this with the vowel in CAT /kæ:n/

But, as you can see, the real important thing is to pronounce or not to pronounce a vowel between K and N.

For more advanced explanations go to the corresponding section in PHONETICS Can / Can't