英语口语中t有时读成d 如later water p 有时读成b 如separation请问是怎么回事

to Forever_zjl, yuzfei and a413864313:
the vowels /p/, /t/, /k/ in front of /s/ are not called "浊辅音", these sounds are never voiced even when they occur before /s/, ok? please don't confuse others if you don't really know phonetics...
we call it /p/, /t/, /k/ before /s/ as unaspirated vowels, they are not equal to voicing vowels like /b/, /d/, /g/, bear it in your mind.
another question about "t有时读成d 如later water", it's called "flap" in linguistic terms. It mainly occurs in American English, but never in Britain English.
if you still have any questions, feel free to ask me.
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yes, the "flap" sound is predictable. as a matter of fact, "flap" is changed from vowel /t/, it only occurs in the situtation that fulfill 2 conditions:
1. the vowel /t/ must be between 2 vowels. eg, city (/t/ between i and y), water (a and e), later (a and e), etc...
(notice the letter t in "separation' is not /t/, it is like a "sh" sound, i can't type the phonetic symbol here, but you check your dictionary.)
2. the syllable of /t/ is not stress. eg. "ty" in "city" is not stress, "ter" in "water" in not stress...
in this situation, the /t/ sound will change to a "flap" sound in American English. (notice only American English... British people think it quite strange actually...)

噢
这个是读音浊化了
T P K
在S前要读成D B G的音
这是规律

s重读时后面的轻辅音变成相对的浊辅音。你举得例子不对。如stupid

楼主说的现象叫做辅音浊化，可是你举的单词恰好不符合浊化的规则，因为在later water 和separation里面都没有读成b和d，