自己做什么自己知道--用英语怎么说

这句话要视情况而定。
抱怨时说 It is my own business.
如果只是简单的陈述句就说 I know what I am doing now.(现在时)/I know what I did.(过去时)/I know what am I going to do.(将来时)

you know what you do

You yourself know what you are doing.

I know what I`m doing.

One knows what he did or will do.

You knew what you did.