Learn Hardcore Russian: На улице тепло.

In Russian, the present-tense form of the verb быть (“to be”) is usually omitted in simple nominal or predicative sentences. So instead of saying На улице есть тепло, you simply say На улице тепло (“It’s warm outside”).

What case is улице in, and why?

Улице is in the prepositional case (singular feminine). The preposition на requires the prepositional case when you’re talking about location (“on/at the street,” i.e. “outside”).

What part of speech is тепло here?

Here тепло functions as a predicative (an impersonal predicate). Grammatically it’s the short neuter form of the adjective тёплый. In some analyses it’s also called an adverbial predicate. But the key point: it describes the state (“warm”) without a subject.

How would I ask “Is it warm outside?” in Russian?

You can simply use rising intonation:
На улице тепло?
Or insert the particle ли for a more formal question:
Тепло ли на улице?

How do I talk about the weather being warm in the past or future?

Add the appropriate tense form of быть:
• Past: Вчера на улице было тепло (“Yesterday it was warm outside”).
• Future: Завтра на улице будет тепло (“Tomorrow it will be warm outside”).

Why isn’t there a word for “the” or “a” before улице?

Russian has no articles like “the” or “a.” Definiteness or indefiniteness is understood from context, so you simply say На улице тепло, and it can mean “It’s warm outside” in general.

Can I use тепло in other expressions the same way?

Yes. You can replace на улице with any location:
• В комнате тепло (“It’s warm in the room”).
• В лесу тепло (“It’s warm in the forest”).
Or switch adjectives:
• На улице холодно (“It’s cold outside”).
• На улице жарко (“It’s hot outside”).