The first answer to this question is the obvious one : can
is the affirmative and can't
the negative. But I suspect that's not what you mean.
Are you actually asking about pronunciation - the fact that in normal speed speech the /t/ sound is often dropped from the utterance? ie in carefully spoken speech I can't go
would be pronounced /aɪ kɑ:nt gəʊ/ (I'm using British pronunciation here) whereas in normal speech, it's frequently /aɪ kɑ:n gəʊ/ - ?
Dropping a sound like this is called elision, and it's very frequent in English, especially affecting the sounds /t/ and /d/ at the end of the syllable. It may happen before a vowel - I can't eat that!
/aɪ kɑ:n i:t ðæt/ or a consonant, as above.
In front of another consonant,/t/ will frequently change into completely different consonants too - for example I can't go might well become /aɪ kɑ:nk gəʊ/, Similarly in an expression like that bag
the /t/ might change to /p/ : /ðæp bæg/. When the consonant changes like this, it's called assimilation.
The reason why elision and assimilation happen is becusae it's easier to pronounce the words that way. Try saying /t/ /g/ and /t/ /b/ in sequence. You'll notice that you have to change the position of your mouth or tongue completely between the two sounds, and there's quite a lot of muscular effort involved. Now try the sequences /k/ /g/ and /p/ /b/ - they're made in the same position and there's consequently much less effort required.
When we're speaking rapidly (as in normal conversation) there isn't always the time to make the necessary changes of mouth position between sounds. So we simplify the sound sequence by either dropping a sound or by changing it to one which is quicker and easier to pronounce together with its neighbour.
Going back to can
, in British English it's easy to distinguish between the two because, even if the /t/ is missing, the vowel is different : can
= /kæn/ (or unstressed /kən/) whilst can't
= /kɑ:nt/. So even without the /t/ it's clear which is intended. American English is a bit more difficult as both words use the vowel /æ/ -/kæn/ and /kænt/.
So, in answer to your question (and I hope that was the question you intended), the /t/ and /d/ sounds at the end of any syllable may frequently disappear or be replaced by a different consonant in normal-speed speech, but there's no "rule" to say that it will or it won't. Personally, I teach my students to recognise when it's happening as they'll need to if they want to understand native speakers (that would probably be /ʌnəstæn neɪtɪv spi:kəz/ !!) but it's not necessary for them to be able to pronounce the words like this themselves - in fact it's often counter-productive. As we've said, they're a consequence of rapid speech. and may learners just aren't fluent enough to reach that sort of speed. Inserting these features just makes them sound really strange.
How nice to have a phonology question for a change