Learn Farsi: اینجا مدرسه است، لطفاً نخندید.

It comes from the verb خندیدن (to laugh). To make a negative command, you take the negative prefix نـ (na-) and add it to the present stem خند (khand). Then, we add the personal ending ـید (-id). Altogether: نـ + خند + ـید = نخندید (do not laugh).

Is the command نخندید meant for one person or a group?

It can be either! The ending ـید (-id) is technically the plural "you" form. However, in Farsi, using the plural form for a single person is the standard way to be polite. Since the speaker is using لطفاً (please) in a public place, they are using the polite form. If speaking casually to a single person, it would just be نخند (nakhand).

Why does the Farsi sentence use اینجا (here) instead of the word for "this"?

When introducing or identifying a place in Farsi, it is much more natural to use اینجا (here) rather than این (this). So, while the English translation says "This is a school," the Farsi literally says "Here is a school."

Why is لطفاً pronounced "lotfan" when it doesn't end with the letter for "n"?