Well, I konw you wanna get a normal rule to express acts. "He was acting foolishly." 他真傻/他表现得真傻。 "表现得" is often used in a little formal situation, especially in news, composes, appraising someone, etc. Due to the culture differeces, we often describe a person with "He is + adj." 他真好。 他很厉害。 他真行。 他真是忘乎所以了才会那样做。 他真是没有一点爱心。 BY BRUCE, SHANGHAI, CHINA

You can also use 扮得 especially when the context also indicates the action is purposefully done as such or with feigned: 詐

I agree with Chloe Lin. If you're looking for a literal translation, his suggestion "表现得" fitted very well. Here are my translations (if you're looking for more inputs): 1. He was acting foolishly. 他的行为很愚蠢。 2. She acts very calm in stressful situation. 她在紧急的情况下一律都显得很镇定。 3. Some people act very childishly. 有些人行为很幼稚。

In these three sentences, I think "to act (in a certain way)" can be translated as "表现得". He was acting foolishly. 他表现得很傻。（Though idiomatically, I'd probably say 他在做傻事） She acts very calm in stressful situation. 她在高压情境下表现得很沉着。 Some people act very childishly. 有些人表现得很孩子气。