Learn Hardcore Russian: Мне хочется тёплого молока перед сном. - I feel like having warm milk before sleep.

Because Мне хочется... is an impersonal construction. Literally it’s like To me it is wanted / it feels like...
So мне is dative case (“to me”), and there is no grammatical subject я in the sentence.

What does хочется mean, and how is it different from хочу?

Я хочу (молоко) = a straightforward, direct statement of desire: I want (milk).
Мне хочется (молока) = a softer, more “craving/feeling like” meaning: I feel like (some) milk.

Хочется is the impersonal form and often sounds more natural for momentary cravings.

Why is it тёплого молока (genitive) and not тёплое молоко (nominative/accusative)?

With хотеться in this impersonal pattern (мне хочется ...), Russian commonly uses the genitive to express an indefinite amount:

Мне хочется тёплого молока = I feel like some warm milk.

Тёплое молоко would sound like you’re naming a specific, definite item (and it’s less typical after хочется).

Is молока singular or plural? Why does it look like “of milk”?

It’s singular. Молоко is a mass noun, and its genitive singular form is молока.
Russian uses the genitive here to mean some amount of milk, similar to a partitive idea.

What case is перед сном, and why?

Перед requires the instrumental case.

сон (sleep) → instrumental сном
So перед сном = before sleep / before going to bed.

Can I also say перед тем как спать or перед тем, как я ложусь спать?

Yes, and they’re common:

перед сном = shortest, most general (before sleep/bedtime)
перед тем как спать = before going to sleep (a bit more explicit)
перед тем, как я ложусь спать = before I go to bed (more specific and personal)

What’s the role of тёплого? Why does it end in -ого?

Тёплого agrees with молока in:

gender: neuter (молоко)
number: singular
case: genitive

So тёплый → genitive neuter singular тёплого.

Do I need to pronounce ё differently from е in тёплого?

Yes. ё is pronounced yo and is always stressed.

тёп- sounds like tyop- (soft t, then yo)
In writing, ё is sometimes replaced by е, but pronunciation remains ё if that’s the word.

What’s the natural word order here? Can I rearrange it?

Мне хочется тёплого молока перед сном is very natural. Variations are possible for emphasis:

Перед сном мне хочется тёплого молока = emphasizes before bed
Мне перед сном хочется тёплого молока = also fine, slightly conversational

But keep мне хочется together in most cases.

Can I add a verb like выпить (“to drink”)?

Yes, and it often sounds even clearer:

Мне хочется выпить тёплого молока перед сном. = I feel like drinking some warm milk before bed.

Without выпить, the sentence is still perfectly correct; it just focuses on the desire for the thing.

How do I negate it?

Use не:

Мне не хочется тёплого молока перед сном. = I don’t feel like warm milk before bed.

This is the normal way to say you’re not in the mood for something.

Is this sentence formal, informal, or childish?

It’s neutral and natural in everyday speech.
Мне хочется... can sound slightly softer/more personal than я хочу..., but it isn’t childish or overly formal.