Learn Hardcore Croatian: Kad imamo mali budžet, tražimo oglas za besplatan događaj u gradu. - When we have a small budget, we look for an ad for a free event in the city.

Both are correct here and mean the same thing.
In everyday speech, kad is more common and sounds more natural. Kada can sound a bit more formal or careful, but it is also very common and not “too formal” to use in conversation.

So yes, you can freely replace kad with kada in this sentence.

Why is there a comma after budžet in Kad imamo mali budžet, tražimo…?

Croatian normally uses a comma between a dependent (subordinate) clause and the main clause.

Kad imamo mali budžet, → subordinate clause (condition / time)
tražimo oglas za besplatan događaj u gradu. → main clause

This is similar to English:

When we have a small budget, we look for an ad…

If you reverse the order, the comma usually disappears in both languages:

Tražimo oglas za besplatan događaj u gradu kad imamo mali budžet.
(We look for an ad for a free event in the city when we have a small budget.)

Where is the word we in this sentence? Why isn’t mi written?

Croatian usually drops subject pronouns because the verb ending shows who the subject is.

imamo = we have
tražimo = we look for

The -mo ending always marks 1st person plural (we) in the present tense. So:

Kad imamo mali budžet… literally: When have-we a small budget…
Tražimo oglas… literally: Look-for-we an ad…

You can say Mi kad imamo mali budžet, tražimo…, but that’s only used if you want to emphasize we, for contrast, like:

Mi kad imamo mali budžet, tražimo besplatne događaje, a oni troše puno novca.
(We, when we have a small budget, look for free events, and they spend a lot of money.)

Why is it mali budžet and not something like malo budžeta?

Both mali budžet and malo budžeta are correct, but they mean slightly different things.

mali budžet = a small budget
- mali is an adjective (small)
- budžet is a countable noun: one budget, two budgets
malo budžeta = a little (amount) of budget
- malo is an adverb/quantifier meaning a little, not much
- budžeta is genitive singular (of budget), after malo

In this sentence, mali budžet is standard and natural, because you’re thinking of having a small budget as a whole, not just “a little bit of budget” as an abstract amount.

What case is budžet in, and why does mali have that form?

In Kad imamo mali budžet, the verb imati (to have) takes its object in the accusative case:

budžet → accusative singular masculine (same form as nominative here)
mali → accusative singular masculine form of the adjective

For masculine inanimate nouns in the singular, the nominative and accusative usually look identical:

nominative: mali budžet (a small budget)
accusative: mali budžet (we have a small budget)

This is why mali and budžet don’t visibly change form, but grammatically they are in the accusative.

What exactly does oglas mean here? Could I use reklama instead?

oglas is a general word for:

an advertisement
a notice, listing, classified ad, announcement

In your sentence, oglas za besplatan događaj is like:

an ad / listing for a free event

reklama is more like commercial advertising, especially:

a TV or radio commercial
promotional advertising

So:

Oglas za besplatan događaj → a listing or notice for a free event (e.g. in a newspaper, online, on a poster)
Reklama za besplatan događaj → an advertisement promoting a free event (e.g. a banner ad, a commercial)

In most contexts here, oglas is the more neutral and natural choice.

Why is oglas singular? In English we might say “we look for ads for free events”.

Croatian can use a singular noun in a general or indefinite sense where English might prefer a plural:

tražimo oglas za besplatan događaj
literally: we look for an ad for a free event
but pragmatically it often means we look for (some) ad(s) for a free event

You could also say:

Tražimo oglase za besplatne događaje.
(We look for ads for free events.)

Both are correct; the original sentence focuses more on finding some (any) ad for a free event, rather than on the number of ads.

What is the function of za in oglas za besplatan događaj?

za is a preposition that usually takes the accusative case and often means:

for, intended for, about

In oglas za besplatan događaj:

za
- besplatan događaj (accusative) → an ad for a free event

Other similar patterns:

karta za vlak – a ticket for the train
poklon za mamu – a gift for mom
brošura za turiste – a brochure for tourists

So za here links the ad and what it is about / for.

Why is it besplatan događaj, not besplatni događaj?

Croatian adjectives often have two forms in the singular masculine:

indefinite (short) form: besplatan
definite (long) form: besplatni

Both can appear before the noun, but there is a nuance:

besplatan događaj – a free event (indefinite, any such event)
besplatni događaj – the free event (more specific / known)

In modern usage, this difference is not always strongly felt, and both may be heard. But in a generic sentence like yours, describing what you usually look for, besplatan događaj (indefinite) is more natural.

Why is it u gradu and not u grad?

The preposition u can use different cases, mainly:

u
- accusative → movement into something
u
- locative → location in something

In your sentence, you are talking about location (in the city), not movement:

u gradu → locative case = in the city

If you said:

Idemo u grad. → accusative = We are going to the city.

So:

oglas za besplatan događaj u gradu
→ an ad for a free event in the city (locative, static location)

Is the present tense imamo / tražimo talking about right now, or about a repeated habit?

In Croatian, the present tense of an imperfective verb (like imati, tražiti) can mean:

Something happening right now
- Sada imamo mali budžet. – Right now we have a small budget.
A repeated, habitual action, or a general rule
- Kad imamo mali budžet, tražimo oglas za besplatan događaj u gradu.
  → Whenever / When(ever) we have a small budget, we look for an ad…

In this sentence, the kad-clause strongly suggests a general, repeated situation: any time you are on a small budget, you do this.

Could I change the word order, for example put u gradu earlier in the sentence?

Yes, Croatian word order is relatively flexible. You can move elements for emphasis or style while keeping the meaning almost the same.

Some possible variations:

Kad imamo mali budžet, u gradu tražimo oglas za besplatan događaj.
(slight emphasis on in the city as the place where you search)
Tražimo oglas za besplatan događaj u gradu kad imamo mali budžet.
(main clause first; feels a bit less “formal” in structure)
Kad imamo mali budžet, tražimo u gradu oglas za besplatan događaj.
(focus on in the city as the location of the search)

The original order:

Kad imamo mali budžet, tražimo oglas za besplatan događaj u gradu.

sounds very natural and neutral in Croatian.