Question

为什么句子i want to do sth不写成i want do sth

Answer 1

因为want to do sth想要做某事，是固定搭配，不能是want do sth。必须用不定式to do来表示目的。所以不能省略to。
want 英[wɒnt] 美[wɑ:nt]
v. 想要; 希望; 打算; 需要…在场;
n. 需要的东西; 缺少; 贫穷;
[例句]I want to say how really delighted I am that you're having a baby.
我想说你怀孕了我有多高兴。

want to do sth例句：
1、I don't want to leave, but I can't go on.
我不想离开，但我继续不下去了。
2、I want to have a talk with him alone.

我要和他单独谈一谈。
3、I don't want to hurry you.

我不想催你。
4、I've been there already, so I don't want to go again.

我早已去过那里了，所以我不想再去了。
5、Yes, I want to share my life with you.

是的，我想和你共度一生。

Answer 2

想要做某事，不能是want do sth。do必须用非谓语形式to do。即：
想要做某事want to do sth

Answer 3

want to do sth是固定搭配