Question

是I think do sth.还是I think doing sth

Answer 1

两个都是错的，应该是i think about sth。think是不及物动词，后面不能直接加动词和动词ing的形式，在使用的时候要直接加名词或者是加上介词，如：about、of。
举例一：
People think they can treat me like dirt!
人们以为可以视我为草芥！
举例二：
It is surprising how many people start sounding off about something without really deciding what they think about it.
居然有那么多人还没有真正确定自己的想法就开始大发议论&这真是令人吃惊。
举例三：
It behoves us to think of these dangers.
我们必须考虑到这些危险。

Answer 2

都不可以。

1. i   think后面是不能直接动词形式，只能接宾语从句。如：i think  he will  come./i don't think he will come.

2. 你是用“想”做某事，就用want  to  do  sth.