Don't digress ( from the subject ) when lecturing.

讲课时不要岔开 ( 主题 ).

辞典例句

His essay digress from the main subject.

他的文章离开主题.

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I'll tell you a funny story, if I digress from my subject for a monent.

如果允许我离题一会儿,我就给你们讲个有趣的故事.

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