I can't do it (all) by myself.我一个人做不下来。

It's hard to believe that I started this company by myself 20 years ago.很难相信20年前我独自一人建立了这家公司。

I went to the movies by myself.我一个人去看电影。

I had to play in my room (all) by myself.我只能独自一人在房间里玩。