In his lecture,he bore down on the importance of idiomatic usage in a language.

他在演讲中着重强调了语言中习法重要性。

'To pull a face' is an idiomatic expression.

'To pull a face'──'愁眉苦脸'是一个习表达法。

In our reading we should always be alert for idiomatic expressions.

我们在阅读过程中应经常注意法。