Chinese · Pronunciation Guide
duh — short, weak, no tone at all. An unaspirated d onto a relaxed back vowel, over almost before it starts.
de
This is the purest neutral-tone syllable in the language: it has no citation tone to lose and never acquires one. Consequently it never participates in sandhi, and it never blocks it either in the way a lexically-neutral syllable such as the shang in 早上 does — it simply comes after everything and adds no weight.
Listen: 的
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这是老师的书。
Zhè shì lǎoshī de shū.
This is the teacher's book.
Listen: 这是老师的书。
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这是老师的书。 shows the particle doing its job between a noun and a noun. Lǎoshī is a third tone plus a first tone, so no sandhi occurs and lǎo keeps its dip; then de clings to the high shī and copies that pitch before shū arrives, also high. So the particle is nearly inaudible as a separate event, which is exactly right.
English speakers give it far too much substance, producing a clear "duh" with a full vowel and equal timing, which makes sentences sound chopped into equal blocks. It should be shorter than the syllables around it and lower in energy. The other habit is aspirating the d into a "tuh" — keep the breath in, and let the sound be produced almost lazily.