It is helpful at this point to introduce two further rules. Nasalization causes the soft palate to be lowered during a vowel before a nasal: the lowering of the soft palate which is required for nasals anticipates the nasal’s oral closure. The rule, which is variable, affects any vowel before a nasal in the same syllable (whether or not ambisyllabic). So it will apply in keen and keener, but not in the first syllable of keynote. Vowels are also partially nasalized after nasals, as in mad, mile, note, knife.
Nasal deletion deletes the oral closure (bilabial for /m/, alveolar for /n/, velar for /ŋ/), so that just a nasal vowel is produced. It applies before a voiceless stop in the same syllable (whether or not ambisyllabic). So it applies in camp, can’t, thank and in camper, central, thanking, but not in campaign, centrality or concord (/ˈkɑːŋkɔrd/ (noun)).
The combined effect of the two rules is therefore:
Vnt becomes v᷈t
Vmp becomes v᷈p
Vŋk becomes v᷈k
Now review the conditions in which aspiration, glottalization, nasalization, and nasal deletion occur. Which of these rules apply to /mp/ in:
As you will have determined for yourself, aspiration applies to /p/ in campaign, since it is unisyllabic and syllable-initial in /(kæm)(ˈpeɪn)/. Glottalization applies to /p/ in camp, because it is unisyllabic and occurs in the coda after a voiced sound. Nasalization applies to all three /æ/’s, since in all three cases it is followed by a nasal in the same syllable. And nasal deletion applies to /m/ in both camp and camper, because in either case /p/ occurs in the same syllable as /m/: /(kæmp)/, /(kæm(p)ər)/. So we pronounce:
Finally, we need to look at the pronunciation of GA /ntn/. The operation of nasalization, nasal deletion, and t-glottaling in words like mountain and sentence will have quite spectacular effects. To take sentence as an example: It will be syllabified as /(sɛn(t)əns)/. Of course, /n/ will be syllabic, so /(sɛn(t)n̩s)/. Nasalization gives [sɛ̃ntn̩s]., and nasal deletion [sɛ̃tn̩s]. Finally, glottaling leads to [ˈsɛ̃ʔn̩s].
The same goes for mountain.
Advice for Dutch learners:
Because of its frequency, it would be a good idea to work on glottaled /t/ first. One way to learn to produce this sound in words like sit is to pronounce /sɪ/ with a very short vowel which is chopped off abruptly as you suddenly hold your breath. The effect is [sɪʔ]. To produce a pre-glottalized stop in a word like sits, you can use the same method: say /sɪ/ but now hold your breath a little longer and then pronounce /ts/: [sɪʔ- ts]. Glottalized stops, which are more common in GA than pre-glottalized ones, may be a little more difficult to pronounce. For a glottalized stop you need to reduce the duration of the glottal closure so that its onset coincides with that of the oral closure.
Remember that in absolute coda-final position glottaled /t/, i.e. [ʔ] is probably more common than glottalized /t/; so try to pronounce a glottal stop in sit, football, it was, let them, atlas, partly and the like. Pay special attention to glottaled /t/ before a syllabic nasal, as in button, certain, mountain: [ˈbʌʔn̩, ˈsɜrʔn̩, ˈmaʊ̃ʔn̩].
Schwa tends to be nasalized and raised considerably in the sequence /-ənt/, which may sound rather like [ɪ̃ʔ]. As a result, the second and third syllable of a word like commitment sound rather like those in mitt and mint, respectively: [kəmɪʔmɪ̃ʔ].
Avoid overgeneralizing glottalization or glottaling to lenis stops: partly is [ˈpɑrtʔli] or [ˈpɑrʔli], but hardly is [ˈhɑrdli]. The vowel is considerably longer in the latter word but, more importantly, there is no glottal closure for /d/: the vocal cord vibration for the vowel /ɑː/ is continued right through to the end of the word.