"Grasp key sentences" can be translated as "Master key phrases" in Engpsh.

This phrase originates from the idea of comprehending and retaining important information in a piece of text, speech, or communication. To "grasp" something means to understand it fully and retain it in one's memory, while "key sentences" or "key phrases" refers to sentences or phrases that carry the most significant meaning or information.

Example in Engpsh:

In order to pass the exam, you must master the key sentences from the textbook.
She quickly grasped the key phrases in the conversation, which helped her to understand the topic better.

Example in Chinese:

为了通过考试，你必须掌握教科书上的重点句子。
她快速地掌握了对话中的重点短语，这有助于她更好地理解话题。