作用(zuò yòng) - effect, function, role in Mandarin

作用 (zuò yòng) is the effect one thing has on another, or the function a part plays in a whole. It is at home in science and in ordinary argument alike: 化学作用 (chemical reaction), 副作用 (side effect), 起作用 (to work, to take effect).

The verb that goes with it is 起, not 有 or 做. Memorise 起作用 as one unit, and its expanded form 起 ... 的作用: 他起了关键的作用 (he played a decisive role). The thing acted upon takes 对: 这个决定对公司有很大的作用.

Keep it apart from 影响. 影响 is influence, usually gradual and often negative, and it works as a verb too. 作用 is the functional effect, mechanical or causal, and it is a noun only. A bad friend has an 影响 on you; a drug has a 作用 on you. Where English says "it had no effect", Mandarin usually says 没有作用 or the blunter 没用.