Learn Hardcore Croatian: Kad budeš kupio mrkvu na tržnici, uzmi i malo luka. - When you buy carrots at the market, also take a bit of onion.

Both kad and kada mean when. Kad is the more common, shorter, everyday form, especially in spoken Croatian. Kada can sound a bit more formal or emphatic, but it’s not “wrong” either:

Kad budeš kupio... = natural, conversational
Kada budeš kupio... = also correct, slightly more formal/explicit

What tense is budeš kupio? It looks like “you will be bought”.

Budeš kupio is Croatian Future II (often described like a “future perfect” idea): “when you have bought / once you buy (and finish buying)”.
It’s commonly used in subordinate time clauses introduced by kad/ako/čim (when/if/as soon as) to show that one action will be completed before another future action.

Structure:

budeš = future form of biti (to be) for ti
kupio = past participle (perfective) of kupiti (to buy)

Why does Croatian use Future II in the kad-clause instead of a “normal future”?

Croatian typically avoids the “plain” future (kupit ćeš) in many time clauses. Instead, it prefers Future II to express completion before another future action:

Kad budeš kupio mrkvu..., uzmi... = first complete buying carrots, then take onions too.

Using Kad ćeš kupiti... is possible in some contexts, but it more often sounds like a direct question (When will you buy...?) or can feel less idiomatic in this kind of instruction.

Why is it kupio (masculine)? What if I’m a woman?

The participle agrees in gender (and sometimes number) with the subject:

(to a man) Kad budeš kupio...
(to a woman) Kad budeš kupila...
(to multiple people, mixed/masc) Kad budete kupili...
(to multiple women) Kad budete kupile...

Budeš/budete stays the same; the participle changes.

Why is it kupio (perfective) and not kupovao (imperfective)?

Kupiti is perfective: it focuses on the action as completed (you successfully buy the carrots). That fits the idea “once you’ve done that, then…”.

Kupovati is imperfective: it suggests an ongoing/repeated process (be buying / used to buy / shop for). In this sentence, the speaker wants the “finished” action first, so perfective is the natural choice.

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

Mrkvu is accusative singular because it’s the direct object of kupio (buy what? carrots).
Dictionary form: mrkva (nominative singular) → accusative singular mrkvu.

Why is it na tržnici and not na tržnicu?

Because na + locative expresses location (being at/in a place):

na tržnici = at the market (location)

na + accusative expresses movement toward a place:

na tržnicu = to the market (destination)

Here, the buying happens “at the market” as a location, so na tržnici (locative) is used.

What is tržnici exactly—what case and what’s the base form?

Base form is tržnica (market, nominative singular).
With na (location), it becomes locative singular: na tržnici.

Why is there a comma after tržnici?

Croatian normally uses a comma to separate a fronted subordinate clause from the main clause:

Kad budeš kupio mrkvu na tržnici, (subordinate time clause)
uzmi i malo luka. (main clause)

In English, you often do the same when the when-clause comes first.

What form is uzmi?

Uzmi is the imperative, 2nd person singular of uzeti (to take): Take!
Other common forms:

uzmite = imperative plural / polite you
uzmimo = let’s take

What does the i mean here, and why is it placed there?

Here i means also / too / as well: “take also a bit of onion.”
It’s placed before malo luka to mark that whole item as an additional thing to pick up. You could also hear uzmi malo luka također, but i is the most common and neutral.

Why is it malo luka—why is luka not luk?

After quantity words like malo (a little), Croatian typically uses the genitive (often called partitive genitive): “a little (of) …”

luk (nominative) → luka (genitive singular)

So:

malo luka = a little (of) onion

Similarly:

puno vode (a lot of water)
malo kruha (a little bread)

Is luk(a) “onion” or “garlic”? I’ve seen both.

Luk most commonly means onion in everyday Croatian (especially in this kind of shopping context).
For clarity, people may specify:

crveni luk = red onion
bijeli luk = garlic (literally “white onion”)

So malo luka here is naturally understood as some onion, unless context suggests otherwise.

How do you pronounce the tricky bits like budeš, mrkvu, and tržnici?

Approximate guidance:

budeš: boo-desh (the š is like sh)
mrkvu: one syllable cluster at the start: mrk- (like a tight mrrk), then -vu
tržnici: trzh-nee-tsee (the ž is like the s in measure)