Learn Hardcore Croatian: Nakon kupnje ću spremiti račun u torbicu. - After shopping, I will put the receipt in my purse.

Nakon is a preposition that normally requires the genitive case.
The base noun is kupnja (purchase), and its genitive singular is kupnje.
So nakon kupnje literally means after (the) purchase.

What exactly is kupnje—is it a verb form or a noun?

It’s a noun: kupnja = a purchase / buying (a verbal noun derived from the verb kupiti = to buy).
In the sentence it behaves like any other noun, just in the genitive because of nakon.

How is the future formed in ću spremiti?

That’s the future tense (Futur I):
ću (1st person singular of htjeti used as a future auxiliary) + infinitive (spremiti).
So ću spremiti = I will put away / I’ll store.

Why is ću in the second position—why not nakon kupnje spremiti ću račun...?

Ću is a clitic (unstressed short word) and in Croatian clitics typically go in the second position of the clause (after the first “chunk”).
Here the first chunk is Nakon kupnje, so ću comes right after it:
Nakon kupnje | ću spremiti...
You can find other word orders in special styles, but this is the standard, natural placement.

Can I also say Spremit ću račun u torbicu nakon kupnje?

Yes. That’s also correct and common: Spremit ću račun u torbicu nakon kupnje.
Meaning stays the same; you’re just moving the time phrase nakon kupnje to the end for a slightly different focus.

What case is račun here, and why?

Račun is the direct object of spremiti (to put away/save), so it’s in the accusative.
For this masculine inanimate noun, accusative = nominative, so it still looks like račun.

Why is it u torbicu (accusative) and not u torbici (locative)?

With u, Croatian uses:

accusative for movement/direction into (put into the purse) → u torbicu
locative for location inside (in the purse) → u torbici

Since you’re putting the receipt into something, you use u + accusative.

What is torbica—is it different from torba?

Yes. Torbica is a diminutive of torba and often means a small bag/purse/handbag.
Torba is more general (bag), while torbica suggests smaller and often more personal (like a purse).

What does spremiti mean here, and could I use staviti instead?

Here spremiti means to put away / to store / to tuck away (often implying putting something where it belongs).
You could also say:

staviti račun u torbicu = to put the receipt into the purse (more neutral, simple “put”)
pospremiti račun = also “put away/tidy away” (often even more “tidying” nuance)

Is there anything about aspect here—why spremiti and not spremati?

Yes:

spremiti is perfective → one completed action (I will put it away as a single event).
spremati is imperfective → ongoing/habitual/repeated (I will be putting away / I will put away (regularly) depending on context).
In this sentence, a single completed action is intended, so spremiti fits best.

How do I pronounce ću and what is the role of the accent mark?

Ć is a “soft” ch sound (similar to something between English t + y in tune for some speakers, but it’s its own sound).
ću is roughly [tɕu].
The mark is not an accent; it’s part of the letter ć, which is different from c and č in Croatian.

Could I drop the subject ja? Should it be Ja ću?

Croatian normally omits subject pronouns because the verb/auxiliary already shows the person.
So Nakon kupnje ću... is fully natural.
You’d add ja (Nakon kupnje ću ja...) mainly for contrast/emphasis (e.g., “I will, not someone else”).