The assimilation of GA /t,d,n,l/ to the place of articulation of a following consonant is probably the most common of the assimilation in GA. They can become bilabial, labiodental, dental, postalveolar and velar, depending on the place of articulation of the consonant that follows. The assimilation under b) and c) below will also occur in formal styles, and should definitely be adopted by the foreign speaker.
a) Alveolar /t,d,n/ become bilabial /p,b,m/ before /p,b,m/.
/t/ becomes /p/ footpath, football, treatment, got better, that pen, that book, that man, it missed
/d/ becomes /b/ deadpan, goodbye, headband, admit, good pens, good books, good men, good people
/n/ becomes /m/ input, sunbath, inmate, unpleasant, one pen, one book, one man, one blank
b) Alveolar /t,d,n,l/ become dental [t̪,d̪,n̪,l̪] before [θ,ð].
/t/ becomes [t̪] eighth, wet through
/d/ becomes [d̪] width, hide that
/n/ becomes [n̪] enthusiasm, in there
/l/ becomes [l̪] although, well then
c) Alveolar /t,d,n,l/ become post-alveolar [t̠,d̠,n̠,l̠] before /r/. Note that /tr,dr/ are post-alveolar affricates.
/t/ becomes [t̠] entry, ratrace
/d/ becomes [d̠] dry, headroom
/n/ becomes [n̠] Henry, in red
/l/ becomes [l̠] ultra, he’ll run
d) Alveolar /t,d,n/ become velar /k,g,ŋ/ before /k,g/. Of course, if /t/ is glottaled, as in [naɪʔˈkæp͜ʔ], there is no assimilation.
/t/ becomes /k/ nightcap, shotgun, that car, that guy
/d/ bcomes /g/ redcoat, mudguard, good cars, good guys
/n/ becomes /ŋ/ income, engage, one car, one guy
e) Alveolar /s,z/ become palate-alveolar /ʃ,ʒ/ before /ʃ/.
Examples are this ship, these ships: /ðɪʃ ʃɪp/, /ðiːʒ ʃips/.
Less commonly, this happens before /j/, as in this year, these years, where you may not want to apply the assimilation yourself, but should be prepared to hear it in the speech of native speakers. If you do use it, be careful not to turn /zj/ into /ʃʃ/ or even /ʃ/ in cases like these years, which at best would assimilate to /ðiːʒ jɪrz/.