Мы не пошли в сауну, потому что после бега мне хотелось только воды.

Пошли is the past tense of пойти, a perfective verb often used for a single trip or the decision to go somewhere.

So Мы не пошли в сауну means we didn’t go / we didn’t end up going to the sauna.

Compare:

• не пошли = didn’t go on that occasion
• не ходили = didn’t go there, often with a more general or repeated-activity feel
• не шли = weren’t on the way / weren’t walking there

In this sentence, не пошли is the most natural choice for a one-time event.

Because the subject is мы = we, and Russian past tense agrees in number.

For past tense:

• я пошёл / пошла
• ты пошёл / пошла
• он пошёл
• она пошла
• оно пошло
• мы пошли
• они пошли

So пошли is simply the plural past form.

Because Russian uses:

• в + accusative for motion into / to
• в + prepositional for location in

So:

• в сауну = to the sauna
• в сауне = in the sauna

Since the sentence is about going somewhere, Russian uses в сауну.

Сауну is accusative singular.

The dictionary form is сауна, a feminine noun ending in -а. For many feminine nouns like this, the accusative singular changes:

• сауна → сауну
• мама → маму
• книга → книгу

So after в with motion, сауна becomes сауну.

Бега is genitive singular.

That is because the preposition после always takes the genitive case.

• после урока = after the lesson
• после работы = after work
• после бега = after running / after the run

The noun is бег = running, so the genitive form is бега.

It is a noun.

Бег means running as an activity. So после бега is literally after running or after the run.

Russian often uses a noun where English might use an -ing form. So instead of a full clause, Russian can just say:

• после бега = after running
• после плавания = after swimming
• после тренировки = after training / the workout

Мне хотелось is a very common Russian way to express a feeling or desire.

Literally, it works like to me, it was desired, but naturally it means:

• I felt like
• I wanted

This construction often sounds a bit softer and more natural for immediate physical or emotional желания, especially things like food, drink, rest, sleep, etc.

So:

• Я хотел воды = I wanted water
• Мне хотелось воды = I felt like some water / I wanted some water

In this sentence, мне хотелось fits very well because the speaker is describing how they felt after running.

Because this is an impersonal construction.

In Russian, impersonal past-tense forms are usually neuter singular, even when the meaning refers to a person.

So in мне хотелось, there is no normal grammatical subject like я controlling agreement. Instead, the person experiencing the feeling is in the dative case: мне.

That is why Russian uses хотелось, not хотел.

The dictionary form is хотеться.

This verb is related to хотеть, but it is used in impersonal expressions such as:

• мне хочется спать = I feel like sleeping
• ему хотелось пить = he felt thirsty / he wanted a drink
• нам хотелось отдохнуть = we felt like resting

So хотелось is the past tense form of хотеться.

Воды is genitive singular, and here it has a partitive sense: some water.

With verbs like хотеть and especially хотеться, Russian often uses the genitive for food, drink, or an indefinite amount of something:

• хотелось чаю = wanted some tea
• хотелось супа = wanted some soup
• хотелось воды = wanted some water

So мне хотелось только воды means something like all I wanted was some water.

Воду is also possible in some contexts, but it feels more like a specific direct object: I wanted water.
Вода would not fit grammatically here.

Только means only or just.

In мне хотелось только воды, it limits what the speaker wanted:

• not food
• not the sauna
• not anything else
• only water

So the idea is all I wanted was water or I just wanted water.

Because потому что introduces a subordinate clause meaning because.

Russian normally separates this kind of clause with a comma:

• Мы не пошли в сауну, потому что...

That comma is standard Russian punctuation.

Yes. That is completely natural.

The main clause is about we:

• Мы не пошли в сауну = We didn’t go to the sauna

The reason clause is about the speaker’s condition:

• мне хотелось только воды = I only wanted water

So the full idea is that the group did not go, and the speaker’s state is given as the reason. English can do the same thing: We didn’t go to the sauna because I only wanted water after running.