Bilo da govorimo o svinji u blatu ili o janjetu na pašnjaku, djeca kažu da je taj dan bio najbolji dio vikenda.

It means whether ... or ....

So:

• Bilo da govorimo o svinji ... ili o janjetu ...
• = Whether we’re talking about a pig ... or a lamb ...

This is a very common Croatian pattern for presenting two alternatives.

Historically, bilo comes from biti (to be), but in bilo da it works as a fixed expression.

So in this sentence, you should learn bilo da as a set phrase meaning:

• whether
• be it that

It does not agree with anything here, and you do not need to think of it as a normal past-tense form in this construction.

Yes. Govorimo is the 1st person plural present of govoriti:

• govorim = I speak / I am speaking
• govorimo = we speak / we are speaking

A useful thing to remember is that Croatian very often uses the simple present where English might use either:

• we speak
• or we are speaking

So govorimo here can naturally mean we’re talking.

Because the preposition o (about) requires the locative case.

So:

• svinja → o svinji
• janje → o janjetu

This is why the noun forms change.

A few notes:

• Feminine nouns in -a often have -i in the locative singular:
  svinja → svinji
• janje is a neuter noun with an irregular-looking stem in oblique cases:
  janje, janjeta, janjetu...

So o janjetu is completely normal.

Because both phrases describe location, and with location Croatian uses the locative after u and na.

• u blatu = in the mud
• na pašnjaku = in/on the pasture

Compare:

• u blatu = in the mud (location)
• u blato = into the mud (movement)

and

• na pašnjaku = on/at the pasture (location)
• na pašnjak = onto the pasture (movement)

So here the locative is used because the pig and lamb are being described as being in a place, not moving into it.

Because Croatian often repeats the preposition with each item in a pair or list, especially when the phrases are a bit longer.

So this is natural:

• o svinji u blatu ili o janjetu na pašnjaku

Repeating o makes the structure clearer and smoother. In English we often omit the second preposition, but Croatian is more comfortable repeating it.

Because djeca is grammatically plural.

It means children, and it is the plural form used for dijete (child). So the verb must also be plural:

• djeca kažu = the children say
• not djeca kaže

This is a common point of confusion because djeca ends in -a, which can look singular to English speakers, but here it is definitely plural.

There are two things happening here:

1. da introduces a subordinate clause

After kažu (they say), Croatian uses da for that:

• djeca kažu da...
• = the children say that...

2. je bio forms the perfect tense

• je = auxiliary is/has
• bio = past participle of biti

Together, je bio is the normal Croatian perfect form. In English, it is often translated simply as was:

• taj dan je bio najbolji dio vikenda
• = that day was the best part of the weekend

So da je taj dan bio... means that that day was...

Because je is a clitic. Croatian clitics usually go very early in the clause, and after da it is very normal to place je immediately after it:

• kažu da je taj dan bio...

This is the standard and natural order.

English speakers often expect the subject to come first, but Croatian word order is more flexible, and clitics follow their own rules.

Here, taj dan is the subject of bio.

The meaning is:

• that day was the best part of the weekend

not

• on that day...

This can be confusing because for masculine inanimate nouns, the nominative and accusative often look the same:

• taj dan can look identical in both cases

But in this sentence, its job is clearly that of the subject, because it is what was the best part of the weekend.

Because bio agrees with dan, which is masculine singular.

So:

• dan → bio
• noć → bila
• selo → bilo
• dani → bili

In Croatian perfect tense, the participle agrees with the subject in gender and number.

Yes. Najbolji is the superlative of dobar (good).

The pattern is irregular:

• dobar = good
• bolji = better
• najbolji = best

So najbolji dio means the best part.

It appears here as najbolji because it agrees with dio, which is masculine singular.

Because dio (part) is normally followed by the genitive to show the whole that the part belongs to.

So:

• dio vikenda = part of the weekend
• dio dana = part of the day
• dio filma = part of the film

That is why vikend becomes vikenda here.