这三个词在语法上有什么特点

need
1 to require sth/sb because they are essential or very important, not just because you would like to have them.就是说需要的人或物是必须的或者很重要的。eg:What do you need your own computer for? You can use ours
2 used to show what you should or have to do.用来表示你必须要做的事eg:All you need to do is complete this form.
want
1 to have a desire or a wish for sth 有主观愿望在里面 eg：She’s always wanted a large family
2 to lack sth 缺少什么东西，这个例子就不举啦。
require
1 to need sth; to depend on sb/sth注意是depend.eg:These pets require a lot of care and attention.
2 常被动语态。~ sth (of sb) to make sb do or have sth, especially because it is necessary according to a particular law or set of rules注意后面的particular law or set of rules
eg：All candidates will be required to take a short test.
有不明白的再追加问题~~~我基本都在线的
require sb to do True marriage requires us to show trust and loyalty
require that The situation required that he be present
require doing Lentils do not require