Learn Hardcore Croatian: Kad budemo završili kupnju, naći ćemo blagajnu bez reda. - When we finish shopping, we will find a cash register without a line.

Kad završimo (kupnju) = a normal present-tense form used with future meaning after kad (very common in everyday speech).
Kad budemo završili (kupnju) = future perfect (often called futur II in Croatian): it emphasizes that one action will be completed before the next action happens. So this sentence highlights the sequence: first we will have finished shopping, then we’ll find a checkout with no line.

What exactly is budemo završili grammatically?

It’s futur II:

budemo = future form of biti (to be) for we
završili = past participle (masculine plural by default for a mixed/unknown group) of završiti (to finish, perfective)
Structure: budemo + past participle.

Why is the participle završili and not završile or završila?

Because the participle agrees with the gender/number of the subject:

završili = masculine plural (also used as the default for mixed groups)
završile = feminine plural (only if the subject is all-female)
završio / završila / završilo = singular forms
If it’s two women speaking about themselves, Kad budemo završile kupnju… is correct.

Is Kad different from Kada?

Mostly stylistic:

kad = more common in everyday speech
kada = slightly more formal / sometimes clearer in careful writing
In this sentence, both work: Kad… / Kada…

Why is kupnju in that form? What case is it?

Kupnju is accusative singular of kupnja (shopping / purchase).
The verb završiti typically takes a direct object in the accusative:

završiti kupnju = finish the shopping.

What does kupnja mean here—shopping or a purchase?

In this context it means shopping (as an activity): finishing your shopping in the store.
But kupnja can also mean a purchase depending on context, e.g. Ova kupnja je bila skupa = This purchase was expensive.

Why is it naći ćemo and not nađemo or ćemo naći?

naći ćemo = future I (we will find), standard word order.
ćemo naći is also correct; it often sounds a bit more emphatic or is used to avoid splitting a clitic chain in some contexts.
nađemo would be present tense; on its own it doesn’t express will, unless used in certain subordinate structures.

What is the verb pair here—why naći and not nalaziti?

Croatian has aspect pairs:

naći = perfective (to find, as a completed result)
nalaziti = imperfective (to be finding / to find repeatedly / to locate)
Because the sentence is about achieving the result “find a checkout,” naći is the natural choice.

Why is blagajnu accusative—what case is it and why?

Blagajnu is accusative singular of blagajna (checkout/cash register).
It’s accusative because naći takes a direct object: you find something → naći blagajnu.

What does blagajna refer to—cashier, cash register, or checkout?

In a supermarket context, blagajna usually means the checkout lane/checkout area (the place where you pay).
It can also refer to the cash register itself, but here “find a blagajna” naturally means “find a checkout.”

What does bez reda mean literally, and is it idiomatic?

Literally: bez = without + reda (genitive of red) = a line/queue.
Idiomatic meaning: with no line / no queue.
Very common phrase in stores: red = the queue of people waiting.

Why is it bez reda (genitive), not bez red or something else?

The preposition bez (without) always requires the genitive case.
So red (nominative) changes to reda (genitive singular): bez reda.

Is the word order fixed? Could I say Kad budemo završili kupnju, naći ćemo blagajnu bez reda in another order?

It’s flexible. Common alternatives:

Kad budemo završili kupnju, naći ćemo blagajnu bez reda. (neutral)
Naći ćemo blagajnu bez reda kad budemo završili kupnju. (focus on “we will find…” first)
Kad završimo kupnju, naći ćemo blagajnu bez reda. (more conversational)
Croatian word order often shifts to match emphasis, while the cases keep roles clear.