反身代词前什么时候用by什么时候不用by

反身代词的用法：
1、 反身代词不能作主语,但是它可以作主语同位语,放在主语后或句末.:如：我亲自去了电影院.误：Myself went to the cinema.正：I went to the cinema myself.
2、 反身代词可以作宾语的同位语.如：I wish I could hear Beethoven himself play this music.我希望我能听贝多芬他亲自弹这首曲子.You can go and ask him himself.你可以去问他本人.
3、 反身代词可以作介词的宾语.如：Nobody,she learnt it all by herself.没人,她自学的.I mended the bike by myself.我独自修的自行车.She said to herself:”what’s wrong with my eyes?”她自言自语地说,“我的眼睛怎么啦?”Don’t always think of yourself.别总是想着你自己.
4、 反身代词作动词的宾语.一般放在像这样的动词的后面：enjoy,help,hurt,teach,wash,look after,get dressed,buy等.如：Did you enjoy yourselves at the weekend?你们周末玩得开心吗?He teaches himself English at home.他在家自学英语.“Help yourselves to some fish .children.”“ 孩子们,随便吃一些鱼 .” He can buy himself a lot of nice things.他可以给自己买好多好东西.
5、反身代词可以作表语.如：I’m not quite myself today.我今天情绪不好.He doesn’t seem himself this morning.今天上午他似乎不舒服.