Breakdown of Прогноз плохой: будет минус двадцать, поэтому вам будет холодно.
плохой
bad
быть
to be
вам
to you (formal/plural)
поэтому
so (therefore)
Questions & Answers about Прогноз плохой: будет минус двадцать, поэтому вам будет холодно.
Why does the sentence say вам будет холодно instead of using вы (you)?
In Russian, physical and emotional states (like being cold, hot, or sad) are expressed impersonally. Instead of saying "You will be cold", Russian literally says "To you it will be coldly." To build this, we use a Dative pronoun (вам) + the future neuter verb (будет) + an adverb (холодно).
Why isn't there a word for "it" before будет минус двадцать?
Russian does not use "dummy" pronouns like English "it" to talk about the weather, time, or temperature. To say "it will be," you simply use the future verb будет all by itself.
Why do we use the adjective плохой for the forecast, but the adverb холодно for being cold?
Плохой (bad) is describing a specific thing—the masculine noun прогноз (forecast)—so it needs to be a standard adjective to match. Холодно, on the other hand, describes an impersonal state or environment rather than a specific noun, so it takes the adverb ending -о.
Is вам plural or formal in this sentence?
Вам is the Dative form of вы. Out of context, it can mean either formal "you" (speaking respectfully to one person) or plural "you all". The impersonal grammar structure is exactly the same for both meanings.