Learn Hardcore Russian: Мне грустно, когда весь день идёт дождь.

Мне грустно literally means “To me it is sad” and is the natural way in Russian to say “I feel sad” (temporary state, how I feel right now).

Мне = to me (dative case, “experiencer” of the feeling)
грустно = sad as a state (not an adjective describing a noun)

Я грустный = I am a sad person (more like a description of character or a more permanent quality), though in context it can also mean “I’m sad now”. Still, Russians most often use Мне грустно for feelings at the moment.

So:

Мне грустно = I feel sad.
Я грустный человек = I am a sad person (by nature / in general).

What exactly is грустно here – an adverb or an adjective?

Formally, грустно is related to the adjective грустный, but in this kind of sentence it functions as a special type of word often called a predicative or category of state word.

It does not modify a verb like an ordinary adverb.
It does not agree with a noun like a normal adjective.

Instead, it acts as the main part of the predicate (like “is sad”):

Мне грустно. – I am (feel) sad.
Ему скучно. – He is bored.
Нам холодно. – We are cold.

So, for learning purposes, treat грустно, скучно, холодно, жарко, стыдно etc. as a special group: “how someone feels / what state they are in.”

Where is the verb “to be” in Мне грустно? Why is there no есть?

In the present tense, Russian almost always omits the verb быть (“to be”) in sentences like this.

Full logical structure: Мне (есть) грустно. – To me (there is) sadness / It is sad for me.
What people actually say: Мне грустно.

This is the same pattern as:

Я студент. (not Я есть студент) – I am a student.
Он дома. (not Он есть дома) – He is at home.

So the verb to be is understood but not spoken in the present tense.

Why do we say идёт дождь? Doesn’t идти mean “to go / to walk”?

Yes, идти normally means “to go / to walk”, but with weather it has a special meaning: “to fall / to be happening”.

идёт дождь = it is raining
идёт снег = it is snowing
идёт град = it is hailing

This is just an idiomatic weather construction in Russian. You can think of it like “rain is going / coming down”, but you should simply memorize:

Идёт дождь. – It’s raining.
Шёл дождь. – It was raining.
Будет идти дождь. – It will be raining.

Can I say Дождь идёт instead of идёт дождь? Is there a difference?

Yes, both are grammatically correct:

Идёт дождь.
Дождь идёт.

They usually mean the same: It is raining.

The difference is mostly word order and slight emphasis:

Идёт дождь. – more neutral, very typical weather phrase.
Дождь идёт. – a bit more emphasis on дождь (“The rain is falling / The rain is coming”).

In your sentence когда весь день идёт дождь, the order идёт дождь is the most natural-sounding.

Why is it идёт (present tense) if we say “all day”? Shouldn’t it be some kind of continuous tense?

Russian does not have a separate continuous (progressive) tense like English (is raining / was raining).

So the simple present идёт covers English “rains / is raining”:

весь день идёт дождь = it rains all day / it is raining all day.

If we are talking about a general situation (whenever this happens), present tense is correct:

Мне грустно, когда весь день идёт дождь.
I feel sad when it rains all day.

If you mean one specific day in the past, you’d change it to past:

Мне было грустно, когда весь день шёл дождь.
I was sad when it rained all day.

What case is весь день in, and why?

Весь день is in the accusative case and expresses duration of time: “for the whole day”.

This is a very common pattern:

Я работал весь день. – I worked all day.
Она спала всю ночь. – She slept all night.
Мы ждали три часа. – We waited for three hours.

So in your sentence:

когда весь день идёт дождь = when it rains for the whole day
Accusative is used to show how long something lasts.

Why is there a comma before когда?

Because когда introduces a subordinate clause (a dependent time clause).

Structure:

Main clause: Мне грустно – I feel sad
Subordinate clause: когда весь день идёт дождь – when it rains all day

In Russian, a subordinate clause introduced by когда, потому что, если, что etc. is normally separated by a comma:

Мне грустно, когда весь день идёт дождь.
Я рад, когда светит солнце.
Я злюсь, когда опаздываю.

Can I put the когда-clause first: Когда весь день идёт дождь, мне грустно?

Yes, and it’s very natural:

Когда весь день идёт дождь, мне грустно.
Мне грустно, когда весь день идёт дождь.

Both are correct and mean the same. The difference is only word order and rhythm, not grammar.

Russian allows flexible word order, especially with clauses like когда…. The comma stays because it still separates the main and subordinate clauses.

What’s the difference between весь день and целый день?

Both can often translate as “all day / the whole day”, and in many contexts they are interchangeable:

Мне грустно, когда целый день идёт дождь.
Мне грустно, когда весь день идёт дождь.

Subtle nuance:

весь день – neutral “the entire day, from beginning to end”.
целый день – often adds a feeling of length / intensity or mild complaint: the whole day (and it was a lot / annoying).

But in everyday speech, people frequently use either, with only a small difference in tone.

Could I say Я грущу, когда весь день идёт дождь instead of Мне грустно…?

Yes, this is grammatically correct:

Я грущу, когда весь день идёт дождь.

Difference in nuance:

Мне грустно… – focuses on your state (“It is sad for me / I feel sad”).
Я грущу… – uses the verb грустить (“to be sad, to grieve”), focuses a bit more on the process / action of being sad and can sound a little stronger or more emotional.

In everyday speech about normal feelings, Мне грустно… is more common and neutral.

Why is it когда and not что (like “that” in English)?

Когда means “when” and introduces a time clause:

когда весь день идёт дождь – when it rains all day

Что means “that / what” and usually introduces a content clause (what someone says, thinks, knows, etc.):

Я знаю, что идёт дождь. – I know that it’s raining.
Он сказал, что придёт. – He said that he’d come.

Your sentence is about time, so you must use когда, not что.