He thinks to be a doctor can help more people.这句话你to be 是什么用法?为什么不用being呢？

He thinks to be a doctor can help more people.
He thinks being a doctor can help more people.
从语法上讲都是正确的,区别在于 to do 形式侧重于将来时态
doing 形式侧重于过去或者现在时态
可以从几个短语中理解
Stop singing 停止唱歌
to sing 停下来去唱歌
forget turning off the light 忘记已经关了灯
to turn off the light 忘记去关灯
你的句子如果侧重于他的理想,表示将来要用
He thinks to be a doctor can help more people.
侧重于现在他的职业已经是医生,就要用
He thinks being a doctor can help more people.

He thinks to be a doctor can help more people.
这句话你to be 是成为的意思
我是从事英语的，乐意为你解答。

think to是词组，认为
to后面加动词原形或be

不定式做从句的主语

to be 作后面的宾语从句中的主语
He thinks （that）to be a doctor can help more people.

不定式做宾语从句中的主语

to表示未发生
be就相当于become