knife是可数。knife的复数是knives。
knifen.刀；v.用刀伤害(或杀害)
She was playing with a knife, so I took it away from her.
她在玩一把刀子，于是我把刀子从她手里夺了过来。
She was playing with a knife, so I took it off her.
她在玩一把刀子，于是我把刀子从她手里夺了过来。
She came at me with a knife.
她拿着刀子向我扑过来。
She went for him with a knife.
她手持尖刀向他刺去。
