A knife. You use 'a' for words that start with a consonant sound. Yes, the 'k' is silent but the following letter, 'n', still has a consonant sound. So you use 'a.' Notice now I say 'consonant sound' and not just consonant. For example you say 'an hour' because the 'h' is silent. And you say things like 'a university' because although it starts with a vowel, it doesn't start with a vowel sound.

Answer : A knife Always use "A" just before the sound of a consonant, here the "n" sound at the beginning of "knife"

Yon should never bring "a" knife to a gun fight.

We use "an" article with vowel sounds, in your case knife sounds "nife" which is not a vowel sound so you cannot say a knife

Thanks for your comment, Alain!