你好：
Nobody know how to run these machines. Know变为Konws
He began to do his homework at eight every evening. begin变为begins
---Why did she angry?---Because he shouted at her just now .did变为was
How many concerts does the pianist give.does变为did
My sister worked in that compeny since she graduated from college three years ago?worked变为has been working
I mailed a letter to you last month .Did you receive it? 后面变为 have you received it?
The train has been here ten minutes.here 和 ten 中间加上 for
I'm looking for my book.l have lostit a moment ago. 去掉后面的 have
Don't get off the bus until it stops. until变为before
Please tell me why you are absent from school yesterday. are变为were
希望能帮到你!