Must, Have to and Need to 必須和需要

 Lina CHU

Must表示有必要進行某些事情的意思，可以用來表達硬性規定或規矩，譬如說，某人想到外國升學，必須參加外語測試。

You must take the language test if you want to study abroad.
如果你想到外國升學，必須參加這個語文測試。(這是規定，必須遵守。)

表達事情的重要性

Must也可以表示某些事的重要性。譬如說，流行性感冒期間，需要經常洗手。

We must wash our hands frequently.
我們需要經常洗手。(這很重要。)

可用Have/Need to等代替

上面的兩種情形，都可以用have to或need to代替must。

You have to take the language test if you want to study abroad.
We need to wash our hands frequently.

否定式意義各不同

Must、have to和need to有否定式(negative)：must not、do not have to、do not need to/need not to，但他們有不同的意思。

Students must wear school uniforms in the inter-school concert.
學生參加校際音樂會時必須穿校服。(規定學生穿校服。)

Students must not wear school uniforms in the inter-school concert.
學生參加校際音樂會時不得穿校服。(規定學生不可以穿校服。)

Must表達硬性規定

Must not與must同樣是硬性規定。

假如學校任由學生穿或不穿校服參加音樂會，應該用do not have to或do not need to(need not)，而不是用must not。

1. Students do not have to(do not need to)wear school uniforms in the inter-school concert.
2. Students need not wear school uniforms in the inter-school concert.
學生參加校際音樂會時不必穿校服。

朋友問："Should I buy her some flowers on the Valentine's Day?"

(「情人節要不要送花給她呢？」)

答案可以是：

"You must." (「你一定要。」)
"You must not." (「你一定不可以。」)
"You do not have to./You do not need to." (「你不一定要。」)

來源 : http://paper.wenweipo.com