。
She cut her hand with a knife.
Literal
She [topic-は] knife [with-で] hand [object-を] cut.
The instrumental で marks ナイフ as the means of cutting — the same で that handles 箸で食べる ('eat with chopsticks'), 鉛筆で書く ('write with a pencil'). The sentence is structurally ambiguous between 'she accidentally cut her hand' and 'she cut her hand on purpose,' but Japanese (like English) defaults to the accidental reading without further context. ナイフ is the everyday loanword for a knife — Japanese also has the native 包丁 (kitchen knife), 短刀 (short blade), and 刃物 (bladed implement), each in its own register and use.