Learn Hardcore Russian: На уроке географии учитель рассказал, как наша планета выглядит из космоса. - In the geography lesson the teacher told us what our planet looks like from space.

на уроке literally: on the lesson, but idiomatically: in class / in the lesson.
в уроке would sound wrong here; it would suggest being physically inside the lesson as an object, which doesn’t fit.

So you say:

на уроке математики / на уроке географии / на уроке русского языка
in math class / in geography class / in Russian class

What case is географии in на уроке географии, and why that ending?

Географии here is genitive singular of география.

Structure:

урок (чего?) географии = a lesson *of geography*

So we have:

на уроке – prepositional case (урок → уроке) after на
географии – genitive, because урок can take a genitive to show the subject of the lesson: lesson of X.

It’s a bit confusing because географии (genitive singular) looks the same as географии (prepositional singular), but here logically it’s genitive: lesson of geography.

Why is there a comma before как in учитель рассказал, как наша планета выглядит из космоса?

The comma is there because как наша планета выглядит из космоса is a subordinate clause (a dependent clause) that completes the meaning of рассказал.

Main clause: учитель рассказал – the teacher told (about something)
Subordinate clause: как наша планета выглядит из космоса – how our planet looks from space

In Russian, when a verb like рассказать, объяснить, показать is followed by a как / что / почему / когда-clause, you normally put a comma before that conjunction.

Could I also say учитель рассказал о том, как наша планета выглядит из космоса? What’s the difference?

Yes, you can. Both are correct but slightly different in feel:

Учитель рассказал, как наша планета выглядит из космоса.
– He told how our planet looks from space. (The как-clause directly completes рассказал.)
Учитель рассказал о том, как наша планета выглядит из космоса.
– He told about how our planet looks from space.

In (2), о том adds a small extra “buffer” word, a bit more formal/explicit:

рассказать о чём? – to talk about what?
о том, как… – about how…

In everyday speech, both versions are common. The version without о том is a bit shorter and more direct.

Why is it как наша планета выглядит из космоса and not как из космоса выглядит наша планета? Is the word order fixed?

Russian word order is more flexible than English. Several orders are possible:

как наша планета выглядит из космоса
как выглядит наша планета из космоса
как из космоса выглядит наша планета

All are grammatically correct. The differences are mainly in emphasis:

как наша планета выглядит из космоса – neutral, focusing on our planet as subject.
как выглядит наша планета из космоса – slightly more emphasis on the verb выглядит.
как из космоса выглядит наша планета – puts more focus on the viewpoint из космоса.

In your sentence, the given order is very natural and neutral.

Why is выглядит used here? Could we say как наша планета смотрится из космоса?

Выглядеть is the standard verb for to look / to appear (visually) in a neutral, descriptive way:

как наша планета выглядит из космоса – how our planet looks from space.

Смотреться can also mean to look, but it often has nuances like to look (in terms of style / impression / suitability):

Это платье хорошо смотрится. – This dress looks good (on you / in general).
Здание странно смотрится среди старых домов. – The building looks strange among the old houses.

You could say:

как наша планета смотрится из космоса – how our planet “looks” from space,

but it sounds more colloquial and slightly more about the impression it makes. For neutral, scientific or descriptive context, выглядит is the best choice.

What case is космоса in из космоса, and why do we use из and not с?

Космоса is genitive singular, required by the preposition из.

из
- genitive = from (the inside of), out of
  - из дома – from the house
  - из города – from the city
  - из космоса – from space

С + genitive usually means from (the surface of/from off of something):

с стола – from (off) the table
с крыши – from (off) the roof
с горы – from the mountain

Space is conceptualized as a “volume” you are in, so из космоса = from space, not с космоса.

Why is it наша планета and not нашу планету?

Because планета is the subject of the verb выглядит, so it must be in the nominative case:

(Кто? Что?) наша планета выглядит… – our planet looks…

If планета were a direct object, then you would see нашу планету (accusative):

Мы изучаем нашу планету. – We study our planet.

In your sentence, наша планета is the thing doing the action (looking a certain way), so nominative is required.

Why is наша used and not наш? What is it agreeing with?

Наша is the feminine form of the possessive pronoun наш (our).

планета is a feminine noun (it ends in -а and is feminine by gender).
Adjectives and possessive pronouns must agree with the noun in gender, number, and case.

So:

masculine: наш дом – our house
feminine: наша планета – our planet
neuter: наше море – our sea
plural: наши города – our cities

Why is рассказал used (perfective) and not рассказывал (imperfective)? What’s the difference?

Рассказать (perfective) vs рассказывать (imperfective):

учитель рассказал – the teacher told / finished telling (completed action, result).
учитель рассказывал – the teacher was telling / used to tell / was in the process of telling (ongoing, habitual, or background action).

In this sentence, we’re talking about a completed action on a specific occasion:

In (that) geography lesson, the teacher told us how our planet looks from space.

So the perfective рассказал is the natural choice.

Could we say На уроке географии учитель рассказывал, как наша планета выглядит из космоса? How would that change the meaning?

Yes, grammatically it’s possible, but the nuance changes:

рассказал – one complete act of telling, seen as a whole event.
рассказывал – emphasizes the process or background:
- He was telling (for some time).
- He used to tell (repeatedly, in general).

Examples:

Вчера на уроке географии учитель рассказал, как наша планета выглядит из космоса.
→ Yesterday, he (once) told us about it.
На уроке географии учитель рассказывал, как наша планета выглядит из космоса, когда директор вошёл в класс.
→ He was telling (in the middle of the story) when the principal walked in.

In isolation (without extra context), рассказал is more natural for a simple narrative statement.

Is выглядит a reflexive verb? It looks similar to verbs like надеяться, бояться, etc.

No, выглядит is not reflexive.

Infinitive: выглядеть – to look / to appear (visually)
3rd person singular: он/она/оно выглядит – he/she/it looks

Reflexive verbs in Russian usually have an ending -ся / -сь in their infinitive form:

надеяться, бояться, мыться, одеваться

Since выглядеть has no -ся, it’s a normal (non-reflexive) verb. The -ит in выглядит is just the regular 3rd person singular ending for many -еть verbs in the present tense.

Can you give the main pronunciation stresses in this sentence?

Yes, here are the main word stresses (stressed syllables in caps):

На урОке геогрАфии учИтель рассказАл, как нАша планЕта вЫглядит из кОсмоса.

More precisely:

на урОке
геогрАфии
учИтель
рассказА́л (stress on the final -ал; often marked as рассказА́л)
нАша
планЕта
вЫглядит
кОсмоса

Correct stress is important in Russian, because changing stress can make a word sound wrong or even like a different word.