Students should not worry about when exactly to use certain expressions.

学生不必担心什么时候使用某些特定的表达。

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Certain expressions - such as in - apply to a variable number of expressions.

某些表达式(比如in)可以应用到多个表达式。

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Further, the effect of certain expressions is undefined. For example, consider the following code.

进一步讲，某些表达式的效果是未定义的。例如，考虑下面的代码。

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You can become aware of and change ineffective body language habits, you can develop a deeper understanding of the impact that certain nonverbal behaviors have on your audiences, and you can add more effective gestures, postures, and expressions to your leadership repertoire.

FORBES: Body Language Will Make Or Break Future Leaders