Learn Farsi: ویزای شما نامعتبر است و باید برگردید. - Your visa is invalid and you must return.

It is formed by adding the negative prefix nâ- to the adjective mo'tabar (valid, reliable). In Farsi, nâ- is a very common way to create a negative adjective, functioning just like the prefix "in-" in the English word "invalid".

Can I attach nâ- to any adjective to make it negative?

Not always. Just like in English where we say "incorrect" rather than "uncorrect", Farsi adjectives take specific negative prefixes. While nâ- is very productive, some words use bi- (meaning "without", like bi-kâr for unemployed) or gheyre- (like gheyre-momken for impossible). You simply have to learn which adjectives naturally pair with nâ-.

Why does vizâ have a ی (y) attached to it before shomâ?

The Farsi word for visa is vizâ, which ends in the long vowel "â". When we want to link it to the pronoun to say "your visa" (vizâ-ye shomâ), we need the connecting vowel called the ezâfe. Because vizâ ends in a long vowel, we must add the consonant ی (y) as a bridge to make it easy to pronounce.

The word bâyad (must) requires the subjunctive, so why doesn't the verb bar-gardid have the subjunctive be- prefix?

You are completely right that bâyad triggers the subjunctive! However, the infinitive here is (to return), which is a compound verb beginning with the directional prefix . In Farsi, verbs that already start with prefixes like , , or almost always drop the subjunctive prefix. So, it simply becomes instead of "be-bar-gardid".