Why are 'student' and 'suspend' not pronounced as written?

I am a Chinese student beginning to learn English. I am curious to know why the word student is pronounced with the sound of d instead of t. Likewise, why is the sound of b used instead of p when people say suspend?

The 't' in student is not pronounced like a 'd'. You just think it is because there is a mismatch between the consonants in the Chinese language and the consonants in English.

Pinyin uses the letter 'd' for an unaspirated voiceless alveolar stop (represented /t/ in IPA), and the letter 't' for an aspirated voiceless alveolar stop (represented /tʰ/ in IPA). English uses the letter 'd' for an unaspirated voiced alveolar stop (represented /d/ in IPA), and the letter 't' for a voiceless alveolar stop, which is either aspirated (/tʰ/) or unaspirated (/t/) depending on its position in the word. Since there are no voiced stops in Chinese, you're not used to distinguishing between stops depending on whether they are voiced and unvoiced, so you don't hear the difference between a /t/ and a /d/, while /tʰ/ sounds different from both of them.

In words beginning 'st', 'sp', and 'sk', I believe all dialects of English use the unaspirated unvoiced stops ('sd', 'sb', and 'sg' in Pinyin), and in words beginning 't', 'p', 'k', they use the aspirated unvoiced stops ('t', 'p', and 'k' in Pinyin). Thus, at the beginning of words, the consonants 't', 'p', 'k' are distinguished from 'd', 'b', 'g' both by being aspirated and by being unvoiced. In the middle of words, things are more complicated; here, the consonants 'd', 'b', 'g' are distinguished from 't', 'p', and 'k' by being voiced.

To explain this, I need to use some technical terminology. Here it is:

• Voiced – pronounced with the vocal folds vibrating. [s] is unvoiced, while [z] is voiced.
• Aspirated – pronounced with a following puff of air. [tʰ] as in English ‘tea’ or Mandarin 他 tā is aspirated.
• Square brackets – [tʰ] – represent sounds as we make and hear them. Slashes – /d/ – represent phonemes, which are the meaningful units that we translate sounds into without thinking. In English, [t] and [tʰ] can be two ways of pronouncing a single phoneme, /t/.

In Mandarin, the distinction between 搭 dā and 他 tā is one of aspiration. The Pinyin spelling, though useful for other reasons, is misleading. In IPA, these are written [ta] and [tʰa].

In English, we may or may not voice ‘d’ at the beginning of a word. So ‘die’ may be either [daɪ] or [taɪ]. We hear [taɪ] as /daɪ/ because we don’t make a distinction. If it were ‘tie’, we would hear [tʰaɪ]. At the beginning of a word, or before a stressed syllable in English, the primary distinction between /t/ and /d/ is aspiration, not voicing. This is the same distinction that Chinese makes.

However, after the letter ‘s’, ‘t’ is not aspirated. The word ‘sty’ is pronounced [staɪ], not [stʰaɪ]. Thus, to the ears of a Mandarin speaker, it sounds like a /b/. It all has to do with the way that our ears translate the sounds into phonemes.

The same phenomena that apply to t/d apply to k/g and p/b.

Furthermore, before unstressed syllables, aspiration also doesn’t happen. In this case, /b g d/ must be truly voiced to make them distinct from the unaspirated versions of /p t k/. So ‘happy’ is [hæpi], while ‘cabby’ is [kæbi]. Notice that the /p/ in ‘happy’ is the same sound as the unaspirated /b/ in Mandarin 笔 bǐ. But in English, it contrasts with /b/ and matches with /p/. It is the unaspirated form of /p/.

One final thing: in American English, /t/ between a vowel is pronounced exactly like /d/. Both become a flap, [ɾ], which is the sound of ‘r’ in Spanish.

The "d" in student is called a 'voiced dental'. It is produced with the tongue pressed up against the roof of the mouth behind the two front teeth. But there is also a sound produced in the vocal cords at the same time as the air is forced out; thus, it is 'voiced'. The 't' at the end of the word is a 'voiceless dental'. The tongue position is the same but there's no vibration of the vocal cords.

Compare dip, tip, dare, tare, seed, seat

This site would be helpful for you: http://ell.stackexchange.com

The initial s and t in student are unvoiced, but the '-ud-' is voiced. Because many native English speakers are a little sloppy in their diction, the vocal chords may start up a fraction of a second before the alveolar plosive and so the t sounds somewhat like d. "sdudent".

This effect is less commonly heard in British English than in American English due to the difference in the vowel sound.