Yes, Aji, the difference between the standard US and UK pronunciations of these words is confusing. In the unstressed or weak form, it's pretty much the same in the positive: kn. In Southern British English, the stressed or strong form of can (Yes I can.) has the same vowel as cat, but stressed or unstressed the vowel in the negative can't is more like the vowel in barn (remembering that in this variety of English r is not pronounced before a consonant). The t in can't is very lightly pronounced and in fact often disappears in connected speech. We mainly go by the difference in vowel sound. English speakers do get confused themselves sometimes and have to ask someone whether they said can or can't.
Bingley

Bingley