USAGE NOTE 用法说明: act and 和 action
You can use action as a countable noun when it means the same as act. 当 action 意思与 act 相同时，可作可数名词:
a kind act 仁慈的行为
a kind action 仁慈的行为
Act is also used in some fixed phrases when it means a particular type of action. act 亦可用于一些固定片语中，指某种特定的行为:
an act of friendship 友谊的表现
an act of war 战争的行为
He was caught in the act (=caught doing something bad). 他被当场捉住。
Act is always countable, but action can be uncountable. act 总是可数的，但 action 可以是不可数的:
We must take action immediately. 我们必须马上采取行动。
What we need is immediate action. 我们需要的是立刻行动。