"Can I get a knife?"
we have plural words what have meaning as 'knife'. But the word 'ナイフ' is very natural in Japanese. It is called a 'ナイフ' what is used with a fork at the time of Western meal.
Besides there is the word '包丁/ほうちょう'. it is used to cut meat, fish and vegetables when cooking. We cut fruit by'ほうちょう' as well. But we also use the knife for cutting fruit. It is smaller than 'ほうちょう'. It is called 'ナイフ'.