already 用于完成时态的肯定句.表示“已经”
例如：I have already finished my homework.
He has done it already.
yet 多用于疑问句和否定句,表示“还”
例如：
Have you finished your homework?
No,not yet.I have not yet finished my homework.
Yes,I have already.
ever 多用于疑问句,表示“曾经”.
Have you ever been to Beijing?
否定句用“never".
I have never been to Beijing.