记

In a sentence

• 我记得今天是你妹妹的生日。

  I remember that today is your sister's birthday.

  Here, 记 is used in the sense of 'recall' or 'remember', embedded in the compound verb 记得.

• 请你记下我的电话号码。

  Please write down my phone number.

  In this sentence, 记 is used as 'note down' or 'record', shown in the verb phrase 记下.

• 你上课要记好老师讲的内容。

  You should take careful notes of what the teacher says in class.

  Here, 记 implies 'to take note of', indicating the action of paying attention to and remembering details.