Learn Arabic (Modern Standard): لا ينس أحد قبعته!

The particle لا here is making a negative command, which puts the following verb into the jussive mood. For verbs that end in a weak letter like the alif maqsura in ينسى, this mood is formed by dropping that final letter entirely, leaving us with ينس.

Can the negative command لا be used with third-person verbs like ينس (he forgets)?

Yes! While you usually see the prohibitive لا used with "you" for direct commands (like "don't forget"), using it with a third-person verb creates an indirect command. Here, لا ينس translates to "Let him not forget" or "He must not forget."

Does أحد mean "no one" or "anyone"?

The word أحد usually means "anyone" or "someone" on its own. However, when it is the subject of a negative or prohibitive sentence (thanks to the لا at the beginning), the combination of the two words translates to "no one" or "nobody."

Why does قبعته (his hat) use the masculine "his" pronoun?

Even though "no one" could refer to people of any gender in English, the Arabic word أحد is grammatically singular and masculine. Because of this rule, any pronouns referring back to it must use the masculine suffix ـه (his).