When communicating in Chinese you may often make mistakes using Chinese grammar like:

False:

    他们上课，我走进教室来了。

    Tāmen shàngkè, wǒ zǒujìn jiàoshì lái le.

True:

    他们上课的时候，我走进教室来了。

    Tāmen shàngkè de shíhòu, wǒ zǒujìn jiàoshì lái le.

句子的原意是，“我走进教室来了”发生在“他们上课”这段时间里。在汉语里应把主谓词组“他们上课”用在“……的时候”的前边，作时间状语。

The sentence means that “my coming into the classroom occurred during the time they were having class”. To express such meaning in Chinese, a subject-predicate phrase such 他们上课 should be used with 的时候 after it, together serving as the adverbial adjunct of time.