Standard English accent (used in most dictionaries) : /ˈsəːt(ə)n/ Standard US accent : /ˈsərtn/ This is from Oxford online. I can imagine some Americans pronouncing the /t/ clearly and others weakening it to something like an /l/.

No, it is never dropped. If you dropped the 't' entirely, the word would sound like 'sern', and nobody would understand what you were saying. The 't' is there, but it is a different sound from a 't' at the beginning of the word. To pronounce 'certain' in a standard American accent, you need to say the first part of the word /sərt/ but leave your tongue touching the roof of your mouth. DON'T release it as if you were going to say 'tea'. Leave your tongue in the same position, and breathe out through your nose as you say 'n'. This makes the 'hn' sound which is typical of the American pronunciation of the final syllable of words like 'mountain' and 'button'.

Thanks for your answers. I checked some materials and found that it is called Glottal Stop. Some words like certain, mountain, beaten, [t] +[n], in spoken English, people will chose pronounce [n]. So it sound like [səːn].

where is it said?... about " t " dropped?