Foreigners often make some strange mistakes in Chinese when they study the most difficult language--- ---Chinese. We try to analyze typical sentences to correct these errors.

         Tāmen shàngkè, wǒ zǒujìn jiàoshì lái le.
False: 他们上课，我走进教室来了。
         Tāmen shàngkè de shíhòu, wǒ zǒujìn jiàoshì lái le.
True: 他们上课的时候，我走进教室来了。

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.