Da nije bilo zastoja na obilaznici, stigli bismo u kamp prije zalaska sunca.

Here da means if, not that.

In this kind of sentence, da introduces a hypothetical or contrary-to-fact condition:

• Da nije bilo zastoja... = If there hadn’t been a delay/traffic jam...

This is a very common way to form unreal conditions in Croatian, especially when talking about something in the past that did not happen.

Because Croatian is using the existential pattern nije bilo + genitive, which means there was not / there were no.

So:

• nije bilo zastoja = there was no traffic jam / there wasn’t a delay

This construction is very common with negation. The noun after nije bilo usually goes into the genitive, which is why you get zastoja instead of nominative zastoj.

A learner can think of it as a set pattern:

• bilo je... = there was / there were
• nije bilo... = there was not / there were no

Because after the negated existential expression nije bilo, Croatian normally uses the genitive.

So:

• nominative: zastoj
• genitive singular: zastoja

That gives:

• Nije bilo zastoja. = There was no traffic jam.

This is one of those patterns that is best learned as a whole chunk: nije bilo + genitive noun

Because na here means location, so it takes the locative case.

• obilaznica = bypass / ring road
• locative singular: obilaznici

So:

• na obilaznici = on the bypass / on the ring road

Compare:

• na obilaznici = on the bypass, located there
• motion would usually look different, for example with na
  • accusative in contexts where something moves onto a surface

Here there is no movement onto the bypass being expressed. It is simply the place where the delay happened.

Stigli bismo is the conditional form of the verb stići.

It is made from:

• stigli = the past participle
• bismo = the conditional auxiliary for we

Together, stigli bismo means:

• we would arrive
• and in this past counterfactual context, more naturally in English: we would have arrived

That is important: Croatian often uses this same conditional form where English uses would have + past participle.

So in this sentence:

• stigli bismo u kamp prije zalaska sunca
• = we would have arrived at the camp before sunset

Because the whole sentence puts the event in an unreal past situation.

The first clause tells us about a past condition that did not happen:

• Da nije bilo zastoja... = If there hadn’t been a delay...

So the result is also understood as a past unreal result:

• stigli bismo = we would have arrived

Croatian does not need a separate form exactly like English would have arrived. The time reference comes from the context of the sentence.

Because stigli is the masculine plural form, which is also the default plural form for a mixed group or an unspecified group.

So if the speakers are:

• a mixed group or unspecified group: stigli bismo
• an all-female group: stigle bismo

This is normal Croatian agreement.

Because Croatian often drops subject pronouns when they are already clear from the verb form.

Here bismo already tells you the subject is we.

So:

• stigli bismo already means we would arrive / we would have arrived

You can add mi for emphasis:

• Mi bismo stigli u kamp...

But it is not necessary in a neutral sentence.

Because bismo is a clitic-like auxiliary and normally does not stand first in the clause.

So if the clause begins with the verb phrase, Croatian naturally says:

• Stigli bismo...

If another word comes first, then bismo can follow that first element:

• Mi bismo stigli...
• Vjerojatno bismo stigli...

So stigli bismo is the natural word order here.

Because u can take different cases depending on whether it expresses movement or location.

Here it shows movement toward a destination:

• u kamp = to/into the camp
  accusative

If you were talking about being located there, you would use the locative:

• u kampu = in the camp

So:

• stigli bismo u kamp = we would have arrived at the camp

This phrase means before sunset.

It contains the preposition prije, which takes the genitive.

So:

• prije
  • genitive

The noun phrase is:

• zalazak sunca = sunset
  literally the setting of the sun

Inside that phrase:

• zalazak becomes genitive: zalaska
• sunce is already dependent on it and appears as sunca

So:

• prije zalaska sunca = before sunset

It describes a hypothetical, contrary-to-fact past situation.

The speaker implies:

• there really was a delay on the bypass
• because of that, they did not arrive at the camp before sunset

So the sentence is not neutral speculation. It strongly suggests the opposite of the condition:

• Da nije bilo zastoja... = If there hadn’t been a delay...
• implication: but there was one

In this sentence, da is the natural and standard choice.

Why?

• da is very common for unreal, counterfactual conditions
• ako is more often used for open, real, or possible conditions

So:

• Da nije bilo zastoja, stigli bismo...
  = natural for If there hadn’t been a delay, we would have arrived...

Using ako here would sound less natural in standard usage for this specific counterfactual past meaning.

A good learner rule is:

• ako = if, in more open/real conditions
• da = if, in unreal/counterfactual conditions like this one