Learn Farsi: در بیمارستان بلند صحبت نکنید.

The verb صحبت کردن (to speak) is a compound verb, meaning the grammar happens on the helper verb کردن. We take its present stem کن (kon), add the negative prefix نـ (na-) to the front, and attach the plural/formal 'you' ending ـید (-id). This gives us نکنید (nakonid).

I thought بلند meant 'tall' or 'high'. Why does it mean 'loudly' here?

You are right! بلند (boland) literally means 'tall' or 'high'. In Farsi, when talking about the volume of a voice or sound, we describe it as being 'high'. So, speaking 'high' simply means speaking loudly.

Why doesn't بلند need a special ending to become an adverb, like the '-ly' in 'loudly'?

Unlike English, Farsi doesn't usually add special endings to turn adjectives into adverbs. The exact same word بلند can mean 'loud' (adjective) or 'loudly' (adverb). Placing it right before the verb makes it clear that it is describing how the action is done.

Is نکنید addressing one person or multiple people?

The ending ـید (-id) is for the plural 'you'. This means it can be addressing multiple people in the hospital. However, Farsi also uses this exact plural form to be polite and formal to a single person, which is the standard way to write public signs and instructions.