1.must和need均可用作名词。must作名词用时，意为“必需的东西，必要做的事”，总是同不定冠词a连用，只有单数形式，没有复数形式。

His new novel is a must for all lovers of crime fiction.

他的新作是所有犯罪小说爱好者的必读书。
The book is a must for middle school students. 这本书是中学生的必读书。

The new 37th issue of National Savings Certificates is a must for any taxpayer.

新发行的第37期国家储蓄债券是任何一个纳税人的必然之选。

A must is a visit to the fascinating and world-renowned Motor Museum.

一定要去参观一下那个举世闻名且精彩有趣的汽车博物馆。

Now this poem is still a must in the school textbooks.

现在这首诗仍然是教科书上的必学内容。

At present, in many parts of China, paper-cutting skill has become a must for women and a symbol of a clever mind.

目前,在中国的许多地方,剪纸技术已经成为女性的必备技能和聪明头脑的象征。

提示
可以说：It is a must. 不可说：They are musts.

2.need作名词用时，既可作可数名词，也可作不可数名词。表示“某种需要，需要的东西，必要的东西”时，为不可数名词。

There is an urgent need for qualified teachers.

迫切需要合格教师。

There is no need for you to get up early tomorrow.

你明天不必早起。

There's a great need of rain for crops. 庄稼急需雨水。
Food is one of our basic needs. 食物是我们的基本必需品之一。
A friend in need is a friend indeed. 患难之交才是至交。