A neednt+完成式常常和could+完成式结合使用。这种用法最好通过例句来说明：
I wanted a copy of the letter，so I typed it twice.
You neednt have typed it twice.You could have used a carbon.
我需要那封信的副本，所以我打了两遍字。
你没必要打两遍。你当时本可以用复写纸打。
I walked up six flights of stairs.
You neednt have walked up;you could have taken the lift.
我爬了六层楼梯。
你没必要走着上去;你本来可以乘电梯的。
She stood in a queue to get an Underground ticket.
But she neednt have stood in a queue.She could have got a ticket from the machine.
她排队买一张地铁车票。
可她本来不需要排队。她本可以在售票机那儿买票。
B neednt have和 should have的比较
should或ought to在以上A的例句中都可以代替need或could：
She shouldnt have stood in a queue.She should have got tickets from the machine.
她本来就不该去排队。她应当在售票机那儿买票。
但这里存在着含义上的差别：
She shouldnt have stood in a queue.
她本来就不该去排队。(她排队是错误的或愚蠢的。)
She neednt have stood in a queue.
她本不必去排队。(这样做是不必要的，但她却排了队。)
shouldnt have(done)有批评之意。
neednt have(done)不含有批评之意。