Leaping boldly into the fray with just what I hear by saying the two words over and over ...
Dear Aji ... welcome to the Board.
Now, one problem you may have is the differences in all the regional accents that Americans have!
As a New Englander with a Boston accent, what I hear is that "can" is a word said in the throat, where "can't" has a nasal quality.
In Maine (also part of New England) I hear the throaty "Can" and the Maine accent uses caint for "can't"
at least it sounds that way.
Good luck with pronunciation ! You have your work cut out for you!
On another point, Americans use facial expression more than Asians, or so I noticed when living in Hawaii, so a bit of observation of the speaker's expression may help you distinguish can from can't.

Finally, hanging with this bunch of "Word Nuts" at AWADtalk will help you with your American/English. So do stick around! And on that note of two Americanisms, ("hanging with" and "stick around") I leave the discussion to the more erudite among us with a respectful bow in their directions.)