關於英文act與action兩者名詞用法差異問題。?

She's a real class act.
此句中的act可以用action取代嗎？
不可以，因為意思完全不同。class act 代表一個精巧而有魅力的人。而class action是一個法律用詞，代表集體訴訟。

當名詞act與action同樣在"行為及行動"的意思上，它們基本上是同義詞，大部份情况下，都可以互用，並不在於"後果"和"進行中"。它們的主要分别，就在於習慣上(idiomatic) 和 配搭上(collocation) 的用法。

以下抄自牛津字典中的解說:
(1) An "act" is usually followed by preposition "of" and/​or used with an adjective; whereas, "action" is not usually used with "of" but is often used with possessive pronouns (his, her, ...); for example:
~ a heroic act of bravery (O)
~ a heroic action of bravery (X)
~ his heroic ​acts during the war (O)
~ his heroic actions during the war (O)
(2) "Action" often combines with the verb "take" but "act" does not:
~ We shall take whatever acts are necessary. (X)
~ We shall take whatever actions are necessary. (O)

1. 請問，此句中的act可以用action取代嗎？
是不行的!因為意思稍有不同

2. 再請問，act與action在當名詞時都同樣有"行為及行動"的意思，請問該怎麼分辨何時要用act何時用action呢？

act著重於 "行為的後果",而action著重於"行為的進行與發生".所以,後者的應用情景多是延續的,長時間的.

3. 例句們:
The search actions last for 3 months.（這句不能用act）
The search acts end up with nothing. 

Applied Usage means idiomatic+ collocation
=Helen, you are a real class act.

Helen, you are a real class act! 
We will take class action against the criminal groups.