The pronunciation of can and can't is probably the best example of the difficulties of English pronunciation for Spanish people. The pronunciation of this (modal-auxiliary) verb presents these features:
- Pronunciation of difficult individual sounds / æ / and /ə/ in can, /ɑ:/ in can't.
- Variations in the way the verbs are pronounced, depending on their position in the sentence: We only pronounce can as / kæn / in short answers, or at the end of a sentence or before a pause, or for emphasis:
A: Can you swim? B: Yes, I can. / kæn / (short answer)
A: Can you have it for Friday? B: I think I can, but I need ... / kæn / (before a pause)
A: Sue can't swim, she's too young. B: Of course she can swim. / kæn / (emphasis)
Can you come at six? /kən/
I can run faster than you. /kən/
- Ellision of sounds: the "t" in can't is very often not pronounced; the "a" in can is very often not pronounced when it is pronounced /ə/ .
- Stress: can't is always stressed, so speakers know if a sentence is affirmative or negative because of the stress on can't, not because they hear "t": I can't go (both "can't" and "go" take the stress)
When can is pronounced /kən/ is always unstressed, and the stress falls on the main verb: I can go. (only "go" takes the stress).
For Spanish speakers it is very difficult not to give stress to any words in a sentence, as our language gives the same stress to all the words. Consequently, when we use can in conversation it is sometimes very difficult for listeners to know if the sentence is affirmative (can) or negative (can't).
There are two tricks we can use to pronounce can in an unstressed position:
1) Omit the vowel sound and pronounce can quickly: I can go to the cinema / ai kn 'gəu /
2) Make can part of the subject, as if they were the same word: I can go to the cinema / aikn 'gəu /
To complete the study of this difficult pronunciation point, you can watch these two videos from Accentworkshop.com. The first one is about the pronunciation of can, and the second about the pronunciation of can't.
If you're still uncertain about this point, Jennifer ESL also has some videos with explanations and examples.
Part 1 http://www.youtube.com/watch?v=qwJAiCj-Rcs&feature=channel
Part 2 http://www.youtube.com/watch?v=B43GFCO4pes&feature=related