Learn Hardcore Ukrainian: Ми йдемо на розпродаж, аби купити капелюх за хорошою ціною. - We are going to the sale in order to buy a hat at a good price.

Йдемо = we’re going on foot (from йти “to walk/go on foot”).
Їдемо = we’re going by vehicle (from їхати “to go/drive/ride”). So your sentence explicitly means walking.

Can I write ідемо instead of йдемо?

Yes. Йти/іти are accepted variants. In modern standard Ukrainian, й- is more common in writing: йду, йдеш, йдемо. You will also hear іду, ідемо in some regions (especially western Ukraine).

Why is it на розпродаж and not в розпродаж?

Ukrainian often uses на with events/activities and institutions: на концерт, на виставку, на роботу. A розпродаж is an event, so на is natural. Use в/у mainly for going into physical spaces (e.g., в магазин).

Why is розпродаж in the accusative case here?

After на with motion (direction), you use the accusative. For inanimate masculine nouns like розпродаж, nominative and accusative look the same in singular. If you were already at the sale (location), you’d use the locative: на розпродажі.

What’s the difference between на розпродаж and на розпродажі?

На розпродаж = to the sale (direction).
На розпродажі = at the sale (location).

Why is there a comma before аби?

Because аби introduces a subordinate clause of purpose. Ukrainian requires a comma before conjunctions like аби/щоб that introduce such clauses.

Аби vs щоб — are they the same?

Both can mean “in order to.” In your sentence, you could say ... , щоб купити ... with no change in meaning.
Register/feel: аби can sound a bit more literary or regional; щоб is the most common neutral choice.
With a different subject, use a finite verb with б/би: Ми йдемо на розпродаж, щоб він купив капелюх.

Why is it за хорошою ціною and not за хорошу ціну?

Both occur:

За хорошою ціною (instrumental) = “at a good price” (very common, idiomatic with qualities/criteria: за низькою/вигідною ціною).
За хорошу ціну (accusative) also exists and is acceptable; you’ll hear it, but many prefer the instrumental with ціна in this meaning. Very natural alternatives: за вигідною ціною, недорого, дешево.

Is по хорошій ціні correct?

You’ll hear по (такій) ціні in everyday speech, but careful/standard style prefers за (такою) ціною. If you’re aiming for textbook-standard Ukrainian, use за ціною.

What’s the difference between йти and ходити?

Йти (imperfective, unidirectional) = go/walk now or in one direction: Ми йдемо на розпродаж.
Ходити (imperfective, multidirectional/habitual) = go/walk habitually or back-and-forth: Ми часто ходимо на розпродажі.

How do I put this in the future or past?

Future, one-time: Ми підемо на розпродаж, аби купити... (perfective піти).
Future, ongoing/planned: Ми будемо йти / ми йтимемо на розпродаж...
Past, ongoing: Ми йшли на розпродаж...
Past, completed/departed: Ми пішли на розпродаж...
Habitual past: Ми ходили на розпродажі...

Can I drop the pronoun ми?

Yes. Ukrainian is pro-drop. Йдемо на розпродаж, аби купити... is fine when the subject is clear from context. Keeping ми can add slight emphasis or clarity.

Can I change the word order?

Yes. Common variants:

Ми йдемо на розпродаж, щоб купити капелюх за хорошою ціною.
Щоб купити капелюх за хорошою ціною, ми йдемо на розпродаж. Word order is fairly flexible and used for emphasis; keep the comma before аби/щоб.

Капелюх vs шапка vs кепка — which “hat” is this?

Капелюх = a brimmed hat (fedora, sun hat, etc.).
Шапка = a beanie/winter hat (no brim).
Кепка = a cap (baseball cap/flat cap). So your sentence is about a brimmed hat.

How does капелюх decline? What’s the plural?

Singular: ном. капелюх, род. капелюха, дав. капелюху, знах. капелюх, ор. капелюхом, місц. капелюсі, клич. капелюше.
Plural: ном. капелюхи, знах. капелюхи (inanimate = same as nominative). Example plural sentence: Ми йдемо на розпродаж, аби купити капелюхи...

Is хороший the best adjective here?

It’s correct and common. Synonyms:

гарний (nice), добрий (good/quality), and very natural for prices: вигідний (advantageous), дешевий (cheap). E.g., за вигідною ціною, недорого.

Any pronunciation tips for tricky letters here?

й in йдемо is like English y in “yes.”
х in капелюх is a voiceless kh (like German “Bach”).
ц in ціною is “ts.”
Don’t confuse йдемо (walking) with їдемо (going by transport; initial sound “yi-”).

Are there other natural ways to say this?

Ми йдемо на розпродаж, щоб купити капелюх недорого.
Йдемо по капелюх на розпродаж. (everyday style; по щось = “to get/fetch something”)
Ми вирушаємо на розпродаж за вигідною ціною на капелюхи. (more promotional tone)

What exactly does розпродаж mean compared to знижка or акція?

Розпродаж = a sale event (clearance/seasonal sale).
Знижка = a discount (percentage off).
Акція = a promotion/deal (buy-one-get-one, limited-time offer).