A needn't+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 needn't 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 needn't 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 needn't 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 needn’t 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 shouldn't 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 shouldn't 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 needn't 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。）

shouldn't have（done）有批评之意。

needn't have（done）不含有批评之意。