The /t/ in negative contractions in English can have three main realisations. In decreasing order of likelihood (all other things being equal):
- it can be a glottal stop
- it can be dropped altogether
- it can be a full [t]
It does not matter at all in negative contractions whether the following sound is a consonant or not in terms of dropping the /t/ altogether. It can easily be a vowel that follows.
So in terms of what native speakers actually do, by far the rarest realisation is with a canonical [t]. However, it is never wrong to use a normal [t] sound. Knowing that a [t] will usually not be present will greatly improve non-native speakers' listening skills though.
The other reason to be aware of the fact that there may not be a [t] present is that it enforces that fact that it is stress which is the most important factor for distinguishing negative contractions from normal auxiliaries. Negative contractions are stressed in English, whereas other things being equal most auxiliaries aren't when occurring in positive sentences. So when trying to distinguish between She can come and she can't come we will listen out for the following rhythms:
The first is what we expect from the positive polarity sentence. The second is the negative.
The final [t] in negative contractions may be affected by the sounds following it.
For example, if the word following the contraction normally starts with [j], as in the first sound in you, then the /t/ and the /j/ may coalesce to form an new affricate sound, /tʃ/. This is the first sound that we hear in words like chair. So the string don't you may be realised as:
- 'doʊntʃu (Gen Am) "donchu"
- 'dəʊntʃu (British RP) "donchu"
Also if the following sound is not alveolar, both the /n/ and the /t/ may change their place of articulation according to the place of the following sound. So for example if the following sound is bilabial, the /nt/ cluster may be realised as /mp/. It is quite common to hear RP speakers saying I cam'p believe it, for example.