"He has a knife!"

Translation: On ma nóż!

[AstroVulpes]

Everbody run!

[PatMiFi]

Why not 'noża'?

[Jellei]

For masculine singular inanimate nouns, the Accusative form looks the same as the Nominative. Therefore, "nóż".

If the sentence was negated and therefore had Genitive, it would be "On nie ma noża!"

[Danny62101]

Mam widelec!

[Brian975267]

Tamto nie jest nóż!

[Jellei]

"To nie jest nóż" :)

I understand that you tried translating "That is not a knife!", but while in English all of the following: this/that/it can be used as a dummy pronoun ("[That/It/This] is not a knife!" all make sense and mean basically the same), in Polish the only dummy pronoun is "to".

[Brian975267]

Ahh, thanks for the tip!