words where it is important to distinguish between the neutral tone and the citation tone
Zhang and Yang (2007:84-89) describe several instances where pronunciations in the neutral tone and the citation tone of a word may coexist:

When contrasting polysemy (辨義). Usually, the neutral tone marks informality or figurative speech.


東西 dōng xi means 'thing; item', but dōng xī means literally the East and West (the cardinal points, or more broadly speaking, the Orient and the Occident).

買賣 mǎi mai means informally 'a transaction', but mǎi mài means 'the collective act of buying and selling'.



When contrasting 'parts of speech' (詞性, lit. the nature of the word). These words are semantically similar (or related). In general, the citation tone is regained when a noun becomes a verb, an intransitive verb becomes transitive, or when the word becomes part of a compound word (e.g., a compound noun, or an idiom). That is to say, the citation tone is needed for complex, derived instances.


差使 chāi shi as a noun means 'an errand runner', but chāi shǐ as a verb means 'to assign (sb. sth.)'.
避諱 (the verb 'to taboo') pronounced bì hui is intransitive, but transitive when pronounced bì huǐ, as in 有所避諱 (having something to apply a taboo on), 避諱名字 (to taboo a name).
利害 (the noun/adjective 'profits and losses') pronounced lì hai is a stand-alone word, but is a constituent of compound words like 利害關係 ('an interest', here the meaning inclines to 利, see 偏義複詞) when pronounced lì hài.


When contrasting formality. These words are semantically identical. The neutral tone is used frequently in colloquial settings.


聰明 (the adjective 'clever'): cōng ming (colloquial) vs cōng míng (formal).
打扮 (the noun 'manner of dressing'): dǎ ban (colloquial noun) vs dǎ bàn (formal noun; or the verb 'to dress up' by rule 2.).


As a result of dialectal differences. In mainland Chinese, some words must be pronounced in neutral tones. In Taiwanese Chinese, however, neutral tones are scarce.


Certain suffixes ～子, ～頭, ～巴 etc.
Directional complements ～上, ～下, ～去 etc.
Particles ～的, ～着, ～一～ etc.


Quasi-neutral tones are permissible in speech to contrast intonation. These words have weakened tones in the face of other stressed components in a sentence. However, they are not documented neutral tones in authoritative exhaustive lists.


家中光景很是慘淡。(hěn shì &gt; hěn shi): in the presence of the stressed adverb 很 indicating the severe extent of poverty, 是 should be read lightly.
你就在此地，不要走動。(zài &gt; zai): in the presence of the stressed adverb 就 indicating the immovability of the listener, 在 should be read lightly.
還說呢，快開始吧。(hái &gt; hai, kuài &gt; kuai): in the presence of the stressed verbs 說 and 開始, indicating the speaker's impatience, adverbs 還 and 快 should be read lightly.

Reference: Zhang Bennan &amp; Yang Ruowei (2007). [普通話連讀音變]. Hong Kong: Commercial Press.