Learn Hardcore Russian: Когда погода хорошая, я выхожу из дома, чтобы погулять и посмотреть на шагомер. - When the weather is good, I leave the house to take a walk and look at my pedometer.

Literally it means “weather (is) good”.
In Russian, after an (often omitted) есть (to be) in the present tense, the predicate is also in nominative.

So:

Погода хорошая. – The weather is good. (both nominative)
Not погода хорошую, because хорошую (accusative) would be used when погоду is a direct object, e.g. Я люблю хорошую погоду. – I like good weather.

Could I say Когда хорошая погода instead of Когда погода хорошая? Is there a difference?

Both are grammatically correct and natural:

Когда погода хорошая, ...
Когда хорошая погода, ...

Difference in nuance is tiny:

Когда погода хорошая – more “neutral” word order; common in speech.
Когда хорошая погода – slightly more “topicalized” хорошая, like “When the weather is good (as opposed to bad)...” It can sound a bit more contrastive or stylistic, but in everyday contexts they feel almost identical.

Why is it Когда погода хорошая and not Если погода хорошая? What’s the difference between когда and если here?

Both are possible, but they’re not identical:

Когда погода хорошая, я выхожу из дома...
Focus: describes a typical, repeated situation. Whenever the weather is good, I go out…
Если погода хорошая, я выйду из дома...
Focus: condition. If the weather is good (this time / in that case), I will go out…

In your sentence, it describes a habit, so когда in the present tense (я выхожу) is very natural.
With если, you’d more likely see a future verb: я выйду.

Why is it я выхожу из дома and not я выйду из дома? What’s the aspect difference?

выхожу – imperfective, present tense, 1st person singular.
выйду – perfective, future tense, 1st person singular.

In your sentence:

я выхожу из дома expresses a regular, repeated action (habit):
I (usually) leave the house / I go out (when it’s nice).
я выйду из дома would express a single, future event:
I will leave the house (if that happens).

Since the sentence describes a recurring behavior “when the weather is good”, the imperfective present (выхожу) is the natural choice.

Why is it из дома and not с дома or от дома? How do these prepositions differ?

Here:

из дома – from the inside of the house (motion out of an enclosed space).
из
- genitive → из дома.

Other prepositions:

с дома – usually “from the surface of the house” (e.g. с крыши дома – from the roof of the house), or in some dialects/older style “from home”, but standard modern Russian prefers из дома for “from the house / out of the house”.
от дома – “from (the vicinity of) the house”, starting at the house and moving away, without the focus on “inside → outside”.

For “leave the house” in the spatial sense, the natural phrase is выйти / выходить из дома.

What form is дома in из дома? I also know дома can mean “at home”.

Right, дома can be two different things:

In из дома
- дома = genitive singular of дом
- After из we use genitive: из дома – out of the house.
In Я дома – I am at home
- дома is an adverb meaning at home.

Same spelling, different functions. Context tells you which is meant:

из дома → preposition + genitive noun
я дома → adverb

Why is чтобы followed by infinitives погулять and посмотреть, and why are they perfective, not гулять and смотреть?

Чтобы here introduces a purpose clause: in order to...

In colloquial Russian, for a simple purpose, it’s very common to use чтобы + infinitive, especially perfective infinitive when you mean a single, complete action as a goal:

чтобы погулять – (in order) to go for a walk (once / for a while)
чтобы посмотреть – (in order) to look / to take a look (once)

If you used гулять or смотреть (imperfective), it would emphasize a process more than a completed goal, and here that sounds less natural. The speaker’s goal is to have a walk and have a look at the pedometer as completed, bounded actions.

What nuance does погулять have compared to гулять?

Both come from the same verb гулять (to walk / stroll / hang out), but:

гулять – imperfective: ongoing process, repeated action.
Я люблю гулять. – I like walking / going for walks (in general).
погулять – perfective with по- often implies:
- to walk for a while,
- to walk once / on this occasion,
- a completed walk.

In чтобы погулять, it suggests:
to go out and have a (nice) walk for some time, not just “to be in the process of walking” abstractly.

Why is it посмотреть на шагомер and not just посмотреть шагомер?

The verb смотреть normally takes на + accusative when you mean look at something:

смотреть на экран – look at the screen
смотреть на небо – look at the sky
смотреть на меня – look at me
смотреть на шагомер – look at the pedometer

посмотреть шагомер can occur in some specific contexts (often with a different nuance, like to check something / examine it), but the standard idiom for look at (with your eyes) is смотреть на + accusative.

Here the natural choice is посмотреть на шагомер – to look at the pedometer.

Do I need to repeat чтобы before посмотреть? Why is чтобы погулять и посмотреть на шагомер okay?

You don’t need to repeat чтобы. In Russian, one чтобы can govern several infinitives joined by и:

Я вышел, чтобы купить хлеб и позвонить другу.
Я учу русский, чтобы читать книги и смотреть фильмы.

Similarly:

чтобы погулять и посмотреть на шагомер

Both погулять and посмотреть share the same purpose marker чтобы. It’s completely natural and idiomatic.

What exactly does шагомер mean, and what gender is it?

Шагомер = шаг (step) + -омер (measuring device).
So it literally means step-measurer → pedometer.

Grammar:

Gender: masculine
Nominative singular: шагомер
Accusative singular (in your sentence): шагомер
(masculine inanimate: nominative = accusative)
With на (direction/target of gaze): на шагомер (accusative)

So посмотреть на шагомер = to look at the pedometer.

Why is there a comma before я выхожу из дома and another comma before чтобы?

Russian comma rules here:

Когда погода хорошая, я выхожу из дома...
- Когда погода хорошая is a subordinate clause (time clause) introduced by когда.
- Subordinate clause + main clause → they are separated by a comma.
..., чтобы погулять и посмотреть на шагомер.
- чтобы погулять и посмотреть на шагомер is another subordinate clause (purpose).
- It’s attached to the main clause я выхожу из дома, so it’s also set off by a comma.

Structure:

[Subordinate time clause] Когда погода хорошая,
[Main clause] я выхожу из дома,
[Subordinate purpose clause] чтобы погулять и посмотреть на шагомер.

How do you pronounce the tricky parts of this sentence? Where are the stresses?

Stresses (stressed syllable in caps):

КОгда (or more commonly: когдА – both heard, but everyday speech often stresses the last syllable)
погОда
хорОшая (ho-RO-sha-ya)
я выхожУ (vy-kha-ZHU)
из дОма
штОбы (SHTO-by)
погуляТЬ (po-gu-LYAT’)
и посмотреть – посмотрЕть (po-smot-RET’)
на шагомЕр (sha-go-MYER)

Spoken as one smooth sentence, many unstressed о’s are pronounced like a (reduction), e.g. погода ~ pagÓda, шагомер ~ shagamér.