Learn Hardcore Croatian: Kad budem došla kući, stavit ću kusur u torbicu i spremiti račun. - When I get home, I will put the change into my purse and put away the receipt.

kad is more conversational and frequent
kada can sound a bit more formal or emphatic in some contexts
In this sentence, Kad is completely natural.

What exactly is the grammar of Kad budem došla kući?

That part uses future II (also called future perfect in some grammars):

budem = present tense of biti (to be) used as an auxiliary in future II
došla = past active participle of doći (to come), agreeing with the subject

Future II is often used after time conjunctions like kad / čim / nakon što to express an action that will be completed before another future action:

When I have come home, (then) I will...

Why is it budem došla and not ću doći?

ću doći is future I and simply means I will come.
But after kad (when) in Croatian, it’s very common to use future II (budem došla) to show sequence:

first: arriving home (completed)
then: putting the change away and saving the receipt

So Kad budem došla kući... emphasizes once I’m home / after I arrive home, rather than just stating a future event.

Why is it došla (feminine)? What would it be for a man or for “we”?

The participle agrees with the subject in gender and number:

feminine singular: (ja) došla
masculine singular: (ja) došao
neuter singular (rare for people): došlo
plural masculine/mixed group: došli
plural feminine: došle

So a male speaker would say: Kad budem došao kući...

What case is kući? Why not u kuću?

kući is a special, fixed adverb-like form meaning (to) home. It behaves like an adverb of direction rather than a regular noun phrase.

Compare:

doći kući = to come home (standard phrase)
doći u kuću = to come into the house (physically enter the building)

So kući is about the destination home, while u kuću is specifically into the house.

Why is there a comma after kući?

Because the sentence begins with a dependent time clause (Kad budem došla kući) followed by the main clause. Croatian, like English, normally separates that introductory clause with a comma:

Kad ... , main clause

If you reverse the order, the comma often disappears:

Stavit ću kusur... kad budem došla kući. (comma usually not used there)

What is stavit ću and why is it written like that?

The full infinitive is staviti (to put). In casual speech and writing, the final -i is often dropped:

staviti ću → commonly written as stavit ću

Grammatically, it’s future I:

stavit ću = I will put

You may also see the more standard-looking spelling stavit ću (common) versus a more formal stavit ću is still accepted in everyday use; the key point is that the infinitive is staviti.

Why is ću separated from the verb (instead of one word)?

In Croatian, future I is normally formed with the clitic auxiliary ću/ćeš/će... placed after the first stressed element, and it is written separately:

stavit ću (not one word)

You can get one-word forms, but mostly with the full infinitive ending in -ti where ću attaches and the -i disappears:

raditi + ću → radit ću (still two words in modern standard writing) Older/alternative spellings and some fixed patterns exist, but the normal modern standard is verb + ću as two words.

Why does the second verb appear as just spremiti without ću again?

Because Croatian can coordinate verbs and keep the auxiliary only once when the subject and tense stay the same:

stavit ću ... i spremiti ... = I will put ... and (I will) store ...

So ću applies to both actions:

I will put the change and (will) put away/save the receipt.

You could repeat it for emphasis or clarity, but it’s not necessary:

stavit ću ... i spremit ću ... (more explicit, slightly heavier)

What cases are kusur and torbicu, and why?

kusur is the direct object of staviti → accusative, but for masculine inanimate nouns, accusative often looks the same as nominative: kusur.
u torbicu: after u with motion/placement into something, Croatian uses accusative.
torbica (little bag/purse) → accusative singular torbicu.

So the structure is:

staviti (što?) kusur = put (what?) the change
u (što?) torbicu = into (what?) the purse

What does torbicu imply compared to torbu?

torbica is a diminutive of torba. It typically suggests:

a smaller bag
often a purse/handbag (depending on context)

So u torbicu feels like into my purse / small bag, whereas u torbu could be a regular bag (shopping bag, shoulder bag, etc.).

Does račun mean “receipt” or “bill” here, and why is it spremiti račun?

račun can mean several things: bill, receipt, invoice, or even account depending on context. In everyday situations after shopping or paying, račun commonly means receipt.

spremiti račun means to put the receipt away / keep the receipt (store it somewhere safe). It’s a common collocation when you want to keep proof of purchase or remember expenses.