Learn Hardcore Russian: Когда на улице темно, мне не хочется выходить из дома. - When it is dark outside, I don’t feel like leaving the house.

Because Когда на улице темно is a subordinate time clause introduced by когда (when). In Russian, a subordinate clause is normally separated from the main clause by a comma:

Когда на улице темно, … = When it’s dark outside, … If you reverse the order, the comma still usually stays:
Мне не хочется выходить из дома, когда на улице темно.

What part of speech is темно here, and why isn’t it тёмный?

темно is a “category of state” word (often treated like an adverb used predicatively) meaning it is dark. It’s used in impersonal sentences about conditions (weather, light, temperature, etc.).

На улице темно. = It’s dark outside. You would use тёмный/тёмная/тёмное as an adjective when describing a noun:
тёмная улица = a dark street
тёмное небо = a dark sky

Why does Russian say на улице (“on the street”) to mean “outside”?

На улице is a common idiomatic way to mean outside / outdoors (literally on the street). It contrasts naturally with дома (at home) and is used even if you’re not literally standing on a street:

На улице холодно. = It’s cold outside.
На улице темно. = It’s dark outside.

Why is it мне не хочется, not я не хочу?

Both can translate as I don’t want to, but they feel different:

Я не хочу выходить… = straightforward, more direct: I don’t want to go out.
Мне не хочется выходить… = more about mood/lack of desire; softer, like I don’t feel like going out / I’m not in the mood to go out. Grammatically, хочется is impersonal: the “person who feels like it” is put in the dative (мне, тебе, ему, etc.).

Why is мне in the dative case here?

Because (не) хотеться works like “it feels desirable (to someone)” rather than “someone wants.” The experiencer is expressed in the dative:

Мне хочется спать. = I feel like sleeping.
Ей не хочется говорить. = She doesn’t feel like talking. So мне = “to me” in structure, though the English translation is “I …”.

What does не хочется literally mean?

Literally, it’s like (to me) it doesn’t feel like it’s wanted / it isn’t desired. In natural English: I don’t feel like… / I’m not inclined to…

Why is выходить an infinitive, and what does it mean exactly?

After (не) хочется, Russian uses the infinitive for the action you (don’t) feel like doing:

(Мне) хочется + infinitive выходить means to go out / to leave (on foot or generally). In this context: to go out (from the house).

Why is it выходить (imperfective) and not выйти (perfective)?

With хочется/не хочется, the imperfective infinitive (выходить) is very common for a general/ongoing idea: “the activity of going out.” Using the perfective (выйти) is also possible, often sounding more like a single, concrete act:

Мне не хочется выходить из дома. = I don’t feel like going out (in general).
Мне не хочется выйти из дома. = I don’t feel like stepping out/leaving (this one time).

What does из дома mean, and why is it из + дома?

из means out of / from inside, and it requires the genitive case.

дом (nom.) → дома (gen.) after из So выходить из дома = to go out of the house / to leave home (from indoors to outdoors). (Contrast: у дома = by/near the house; в доме = in the house.)

Could I also say Когда темно на улице…? Is the word order flexible?

Yes, word order is flexible, and both are natural:

Когда на улице темно, …
Когда темно на улице, … The first is slightly more neutral/typical. The second can add a tiny bit of emphasis to темно (“when it’s dark…”).

Is на улице темно a complete sentence on its own?

Yes. It’s an impersonal sentence describing a state:

На улице темно. = It’s dark outside. Russian often uses this kind of structure for conditions:
На улице тепло/холодно/ветрено/сыро/светло.

Does не go before хочется only, or can it move?

In this sentence, не normally sits directly before the verb: мне не хочется. Moving it would be unusual or would change the structure. If you want to negate a different part, Russian typically uses different wording rather than moving не around.