Understanding the Modal Adverb 何必

The Chinese modal adverb 何必 (hé bì) is used to express a sense of unnecessary action or to question the need for something. It conveys a tone of dismissal or suggests that a certain action or attitude is excessive or unwarranted.

In the examples provided:

1. 我只是和你开玩笑，何必当真呢？
   (Wǒ zhǐ shì hé nǐ kāi wánxiào, hé bì dāng zhēn ne?)
   Translation: "I was just joking with you, why take it seriously?"
   Here, 何必 emphasizes that there is no need to take the joke seriously, suggesting that the reaction is overly serious.

2. 咱们是老同学，何必这么客气。
   (Zánmen shì lǎo tóngxué, hé bì zhème kèqì.)
   Translation: "We are old classmates, why be so polite?"
   In this case, 何必 indicates that being overly polite is unnecessary given their long-standing relationship.

Overall, 何必 is often used in conversational contexts to soften a statement while implying that the other person's actions or feelings are excessive or unwarranted. It can help to create a more casual and friendly atmosphere in dialogue.