In order to comprehend the mechanisms of assimilation some comprehension of the production of speech sounds is needed: Roach, P., English Phonetics and Phonology (4th ed.), Cambridge, CUP, 2010, is recommended.
The most common form involves the movement of place of articulation of the alveolar stops /t/, /d/ and /n/ to a position closer to that of the following sound. For instance, in the phrase ten cars, the /n/ will usually be articulated in a velar position, /ˈteŋ ˈkɑːz/ so that the organs of speech are ready to produce the following velar sound /k/. Similarly, in ten boys the /n/ will be produced in a bilabial position, /ˈtem ˈbɔɪz/ to prepare for the articulation of the bilabial /b/.
|Before a bilabial|
|/n/||/m/||ten men /tem ˈmen/|
|/d/||/b/||bad boys /bæb ˈbɔɪz/|
|/t/||/p/||what presents /wɒp ˈpresənts/|
|Before a velar|
|/n/||/ŋ/||green grass /ˈɡriːŋ ˈɡrɑːs/|
|/d/||/g/||good girl /gʊg ˈgɜːl/|
|/t/||/k/||that kid /ðæk ˈkɪd/|