Learn Hardcore Russian: Я сказал другу, что идёт дождь.

Because the verb сказать takes its recipient (the person you tell something to) in the dative case. You say something “to” someone, and Russian marks that “someone” with the dative.

Why is there a comma before что?

The comma separates the main clause (Я сказал другу) from the subordinate clause (что идёт дождь). In Russian, you always put a comma before что when it introduces indirect speech or a content clause.

What exactly does что do in this sentence? Is it like English “that”?

Yes. Here что is a conjunction meaning “that,” linking the act of saying to the content of what was said (“that it’s raining”).

Why do we say идёт дождь instead of using a verb like льёт or пошёл?

идёт дождь is the neutral, impersonal way to express “it’s raining” (ongoing rain) in Russian.

льёт (“it’s pouring”) is more colloquial and emphasizes heavy rain.
пошёл (“it started to rain”) focuses on the moment rain began, not the fact that it’s currently happening.

Why is дождь in the nominative case?

In the impersonal construction идёт дождь, “дождь” is the logical subject of the verb идёт, so it stays in the nominative case.

Why use the perfective past сказал instead of the imperfective говорил?

Using the perfective verb сказать (сказал) highlights that the action of telling was completed. If you said Я говорил другу, что идёт дождь, it would imply an ongoing or habitual telling, or that the focus is on the process of talking.

Can I omit что and say Я сказал другу идёт дождь?

No. In Russian you need the conjunction что to introduce an indirect speech clause. Without it the sentence becomes ungrammatical.

Could I add a possessive and say Я сказал своему другу, что идёт дождь?

Yes. Adding своему simply clarifies “my friend,” but isn’t mandatory if the context already makes that clear.

Can I change the word order to Я другу сказал, что идёт дождь?

Absolutely. Russian has flexible word order. Moving другу before сказал is perfectly fine and only slightly shifts the emphasis (highlighting “to my friend” more).