Here is a big one for many students, and something to keep in mind if they are
going to the US. If you listen carefully to the way in rapid speech Americans
typically differentiate between stressed/emphasized 'can' and 'can't' it's
mostly the timing and a glottal of some sorts that adds to that timing. I've had
students here in Japan who couldn't understand when Americans were saying, I
can't go OR I can go. When the 'can' of 'I can go' gets no stress, you could say
it's simply c'n but when it gets prominence its a full blown nasal [kae:n]. But
they couldn't distinguish can from can't and that is dangerous. Helpful is that
at least some of the times, there is also a 'yes' or 'no' going with the reply.
But remember, Japanese and a lot of other languages don't use 'yes' or 'no' in
quite the same way.

E.g.,

A: (if it were in Japanese): You mean you can't go?
B: Yes, I can't go.

So we see still yet again multiple factors affecting intelligibility and
comprehension. My cat seems to understand me better when I say yes or no than my
students--perhaps it's the emotional bond?