Na vagi su najprije bile kruške, a poslije i jedna vrećica bresaka.

Because na vagi is in the locative case, used after na when it means on / at a location.

• vaga = scale
• na vagi = on the scale

Compare:

• staviti na vagu = to put onto the scale → movement, so accusative
• biti na vagi = to be on the scale → location, so locative

Here the sentence describes where something was, not motion, so na vagi is correct.

This is the Croatian past tense.

It is made with:

• the present tense of biti (sam, si, je, smo, ste, su)
• plus the past active participle (bio, bila, bilo, bili, bile)

So:

• su bile = they were

The participle bile agrees with kruške, which is:

• plural
• feminine

So the form must be bile.

Here kruške is nominative plural, because it is the subject of the clause.

The clause is basically:

• kruške su bile na vagi = the pears were on the scale

For many feminine nouns in Croatian, the nominative plural and accusative plural look the same, so learners often wonder which one it is. In this sentence, it is nominative because the pears are the thing that were somewhere.

Because su is a clitic, and Croatian clitics usually go in the second position of the clause.

Here the first unit is Na vagi, so the clitic su comes right after it:

• Na vagi su najprije bile kruške.

This is very normal Croatian word order.

A learner-friendly way to think about it is:

• first chunk: Na vagi
• second-position clitic: su
• then the rest: najprije bile kruške

Najprije means first, at first, or first of all.

In many contexts, it is very close to prvo. Both can often be translated as first.

Examples:

• Najprije su bile kruške.
• Prvo su bile kruške.

Both are natural.
Najprije can sound a little more like first, before anything else / initially, while prvo is often the more everyday short option. In this sentence, either would work.

Poslije means afterwards, later, or after that.

So the contrast is:

• najprije = first
• poslije = later / afterwards

This helps organize the sequence of events or states on the scale.

Here i means also / too, not just and.

So:

• a poslije i jedna vrećica bresaka
  = and later also one bag of peaches

It adds another item to what appeared on the scale.

Without i, the sentence would still work, but i emphasizes that this was an additional thing.

Because after a container noun like vrećica (small bag), Croatian often uses the genitive for what is inside it.

So:

• vrećica bresaka = a bag of peaches
• čaša vode = a glass of water
• kutija keksa = a box of biscuits
• vreća krumpira = a sack of potatoes

Here:

• vrećica is the main noun
• bresaka is genitive plural, meaning of peaches

Bresaka is the genitive plural form of breskva (peach).

Very roughly:

• singular: breskva
• plural nominative: breskve
• plural genitive: bresaka

Since vrećica requires the of peaches structure, Croatian uses the genitive plural:

• jedna vrećica bresaka

Vrećica is a diminutive of vreća.

So:

• vreća = sack / large bag
• vrećica = small bag / little bag

In everyday English, you might simply translate vrećica as bag, but grammatically it has that small or smaller nuance.

Yes, that full version is completely possible:

• a poslije je bila i jedna vrećica bresaka

In the original sentence, the verb is omitted because it is understood from the first clause. This is very common in Croatian.

So the full logic is:

• Na vagi su najprije bile kruške
• a poslije je bila i jedna vrećica bresaka

But Croatian often leaves out repeated material when it is obvious.

Because a often connects two clauses with a slight contrast or shift, something like:

• and
• while
• whereas
• and then

Here it marks a change in what was on the scale:

• first pears
• later a bag of peaches

Using a is very natural because the sentence is contrasting one stage with another.

Yes. Croatian word order is flexible, though not random. For example, these are all possible with slightly different emphasis:

• Najprije su na vagi bile kruške, a poslije i jedna vrećica bresaka.
• Na vagi su bile najprije kruške, a poslije i jedna vrećica bresaka.
• Kruške su najprije bile na vagi, a poslije i jedna vrećica bresaka.

The original puts early focus on Na vagi and follows normal clitic placement. It sounds very natural.

Because the subject of the second clause would be jedna vrećica, which is:

• singular
• feminine

So the verb would have to agree with vrećica, not with bresaka.

That gives:

• je bila jedna vrećica bresaka

not:

• su bile jedna vrećica bresaka

Even though there are multiple peaches inside the bag, the grammatical subject is one bag.