knife的复数形式为knives。
knife遵循可数名词复数变化规则：以f或fe 结尾的名词，先去f，fe，再加ves。half-halves、knife-knives、leaf-leaves、wolf-wolves、wife-wives、life-lives、thief-thieves。
另外有些单词以f或fe 结尾加s，例如：belief-beliefs、roof-roofs、safe-safes、gulf-gulfs。
knife的意思：
n.
1、小刀；餐刀；菜刀。
2、短刀；匕首。
3、（机械工程）刃部；（机器上的）刀具，刀片。
4、（外）手术刀。
vt.
1、拿小刀切；拿尖刀戳。
2、（口语）秘密打击；用阴险手段击败；背叛。
3、刀切似地穿过。
4、用括刀涂（颜料等）。
vi.
（刀似地）劈开，穿过。
例句：
1、Deng said she pulled out a knife after the men repeatedly dragged her back as she tried to leave.
邓玉娇说她极力想离开，那几名男子却一再把她往后拉，她就拔出了刀子。
2、Calmly, the man withdrew a small knife from his pocket and carved a mark on the boat at the spot where he dropped the sword.
那个人不慌不忙地从衣袋里取出一个小刀，在船舷上落下宝剑的地方刻了一个记号。
3、There was blood on the painted feet, as though the thing had dripped-blood even on the hand that had not held the knife.
画中人那脚上还有血迹——甚至好像连没有拿过刀的手也在淌血。
4、He went to a rotten log near at hand and began to dig under one end of it with his Barlow knife.
他走到近处的一根烂树干旁边，开始用他的巴露折刀在一头开挖起来。