一些英语问题1.I don't think I can do the job myself.可以加个by么?in呢?I

一些英语问题
1.I don't think I can do the job myself.
可以加个by么?in呢?I don't think I can do the job by myself.
为什么不能呢?
2.我认为数学不如英语重要.
I don‘t think math is（） （） （） English.
①I don‘t think math is more important than English.
②I don‘t think math is as important as English.哪个对?
3.Sam has shorter hair than his brother does.这句话有语病么?怎么改?
4.He is fatter than another one.有语病么?怎么改?