I see now. I misunderstood the question. The glottal stop is a feature of some English accents, including Cockney. It replaces the /t/ sound but never the /d/ sound. It is not a feature of received pronunciation or of the standard southern England which is what you will probably hear in dictionary audio files. Here's a sample of the English soap opera "EastEnders". https://www.youtube.com/watch?v=TTB_7z2nJM4 You can hear the glottal stop used by the black lady in the words: "humiliating", "ante-natal", "daughters", "don't" "got". The white lady has a standard southern England accent. What do you think of these accents?

Maybe you're asking about the 'glottal stop' This is not done by all English speakers by any means. It is certainly extremely annoying to hear in American English. It's kinda of a trendy thing to do among young people in the U.S. at the moment. In summation, pronounce the letters, don't swallow them. It sounds much, much better and is the correct way to pronounce in AmE!

I think I got you Ruiqi. Yes these words are usually pronounced "gar-en" "bur-en" "moun-ain". I don't think I can explain the rules but yes, you "swallow" the T sound in "certain". Other examples could be "impor(t)ant", "di(d)n't", "go(tt)en".

I think you are talking about the schwa which is represented as /ə/ in phonetic transcriptions of dictionaries. The schwa is a feature of all English accents. The pronunciation of any sound cannot be taught in a message like this. But in my view, the first rule is to pronounce a word as it is written phonetically in the dictionary, and as you hear it said by a native speaker. If you do that every time, you will always be understood.

No, I can't agree. I certainly don't pronounce things that way and neither do most of us. This seems to be something imported from England maybe. I can't imagine any native speaker teaching that the glottal stop is the correct way in AmE.