Learn Hardcore Polish: Jesienią pogoda w mieście szybko się zmienia. - In autumn the weather in the city changes quickly.

Jesienią – the most natural, standard way to say “in autumn”.
Na jesieni – also used, but sounds a bit more colloquial or regional; often used in spoken language, especially in some areas.
W jesieni – grammatically possible but unusual and stylistically awkward in modern Polish; generally avoid it.

For a learner, Jesienią is the best and safest choice.

What case is w mieście, and why isn’t it w miasto?

Mieście is the locative singular of miasto (city).

The preposition w (“in”) normally takes the locative when it means location “inside” something:

w mieście – in the city
w domu – at home / in the house
w szkole – at school / in the school

So w mieście literally means “in the city”.
W miasto would use the accusative and would suggest movement into the city (e.g. iść w miasto in colloquial speech, “go out into the city”), but that’s a different meaning and structure.

Why is się used with zmienia? What does się zmienia mean?

The verb zmieniać means “to change (something)”:

On zmienia plany. – He changes (his) plans.

When you add się, zmieniać się becomes “to change” intransitively, like “to change, to be changing, to change by itself” – there’s no explicit object:

Pogoda się zmienia. – The weather is changing / changes.

In this sentence:

zmienia – 3rd person singular, present tense of zmieniać
się – reflexive/“middle voice” particle making it “change (by itself)”

So szybko się zmienia = “changes quickly / is changing quickly”.

Could we say Pogoda w mieście szybko zmienia się instead of szybko się zmienia?

Yes, both are grammatically correct:

Pogoda w mieście szybko się zmienia. (your sentence)
Pogoda w mieście szybko zmienia się.

The first one (szybko się zmienia) is more natural and common.
Polish word order is flexible, but się usually stays close to its verb, and the most neutral orders are:

Pogoda w mieście się szybko zmienia.
Pogoda w mieście szybko się zmienia. ← most typical

What is the aspect of zmienia and how would I say “will change”?

Zmienia is from the imperfective verb zmieniać (“to change / to be changing”).

Pogoda (się) zmienia. – The weather changes / is changing.

To talk about a single future change (will change), you use the perfective verb zmienić:

Pogoda w mieście szybko się zmieni. – The weather in the city will change quickly.

So:

zmieniać (imperfective) – ongoing / repeated action (present, or habitual future with będzie
- infinitive)
zmienić (perfective) – completed action (future: one change)

Is szybko an adverb? How is it related to szybki?

Yes, szybko is an adverb meaning “quickly, fast”.
It’s derived from the adjective szybki (“quick, fast”).

This is a very typical pattern in Polish:

szybki (quick) → szybko (quickly)
wolny (slow / free) → wolno (slowly)
głośny (loud) → głośno (loudly)

So szybko się zmienia = “changes quickly”.

Why is there no word for “the” in pogoda? How do you say “the weather” vs “weather” in Polish?

Polish has no articles (no equivalent of “a/an/the”), so pogoda can mean:

weather
the weather
sometimes even “this weather”, depending on context and intonation

In:

Jesienią pogoda w mieście szybko się zmienia.

English needs “the weather”, but Polish just uses pogoda.
If you really need to emphasize a specific weather, you can add a demonstrative:

Ta pogoda – this (particular) weather
Taka pogoda – such weather

But most of the time, plain pogoda is enough.

Can the word order change? For example, can I say Pogoda w mieście jesienią szybko się zmienia?

Yes. Common, natural variants include:

Jesienią pogoda w mieście szybko się zmienia. – In autumn, the weather in the city changes quickly.
Pogoda w mieście jesienią szybko się zmienia. – The weather in the city changes quickly in autumn.
Pogoda jesienią w mieście szybko się zmienia. – The weather in autumn in the city changes quickly.

Polish word order is flexible; moving jesienią changes the focus slightly but not the basic meaning.
Starting with Jesienią emphasizes the time frame (“As for autumn…”).

Can I just say Jesienią pogoda szybko się zmienia and drop w mieście?

Yes, absolutely:

Jesienią pogoda szybko się zmienia. – In autumn, the weather changes quickly.

By adding w mieście, you specify where this is especially true:

Jesienią pogoda w mieście szybko się zmienia. – In autumn, the weather in the city changes quickly.

So w mieście just narrows down the context; grammatically it can be omitted.

How do you pronounce Jesienią and się?

Approximate pronunciation for an English speaker:

Jesienią: [ye-SHEN-yon]
- J – like English y in yes
- ś (written si before a vowel) – like the sh in she, but a bit softer and more “fronted”
- -nią – similar to nyon: a ny sound plus a nasalized o
się: roughly [shyeh]
- ś (here written s
  - i) – soft sh sound
- ę – nasal e, often sounds close to “e” + a hint of n” at the end

Native pronunciation is more subtle, but [ye-SHEN-yon] and [shyeh] are close enough for learners.

What tense is zmienia exactly? Does it mean “changes” or “is changing”?

Zmienia is present tense. Polish has one present form that can correspond to both:

English simple present: The weather changes quickly in autumn.
English present continuous: The weather is changing quickly in autumn.

Context decides how you translate it.
So:

Pogoda szybko się zmienia.
- can be “The weather changes quickly” (general fact)
- or “The weather is changing quickly” (right now)

Both are correct translations depending on what you want to emphasize.