U vrtu je bila mala žaba kraj lokve.

Because u can take different cases depending on meaning.

• u + accusative usually shows movement into something:
  
  • Idem u vrt. = I’m going into the garden.
• u + locative usually shows location in something:
  
  • U vrtu = in the garden

In U vrtu je bila mala žaba kraj lokve, the frog is already located there, so Croatian uses the locative: vrtu.

Vrtu is locative singular of vrt (garden).

The noun vrt is masculine, and its locative singular form is vrtu.

So:

• nominative: vrt
• locative: u vrtu

Je bila is the past tense of biti (to be) for a feminine singular subject.

The subject is mala žaba (a small frog), and žaba is a feminine noun, so the past participle must also be feminine:

• masculine: bio
• feminine: bila
• neuter: bilo

So:

• On je bio = He was
• Ona je bila = She was

Here, because žaba is feminine, Croatian says je bila.

Because adjectives in Croatian must agree with the noun in gender, number, and case.

Žaba is:

• feminine
• singular
• nominative here

So the adjective mali (small) must take the matching feminine singular nominative form:

• masculine: mali
• feminine: mala
• neuter: malo

That is why we get mala žaba.

Because žaba is the subject of the sentence.

In U vrtu je bila mala žaba kraj lokve, the thing that was there is the frog. Subjects are normally in the nominative case.

So:

• mala žaba = nominative subject
• vrtu = locative after u
• lokve = genitive after kraj

Because kraj meaning near / beside / by is followed by the genitive.

The base noun is lokva (puddle). Its genitive singular is lokve.

So:

• nominative: lokva
• genitive: lokve

That gives:

• kraj lokve = by / beside the puddle

Yes. Those are all common ways to express beside / near.

• kraj lokve
• pokraj lokve
• pored lokve

All are natural, though style and regional preference may vary a little. A learner should recognize all three. In this sentence, kraj lokve is completely normal.

Because Croatian has no articles like English a or the.

So žaba can mean:

• a frog
• the frog

The exact meaning depends on context.

That means mala žaba can be understood as:

• a small frog
• the small frog

If the meaning shown to the learner is a small frog, that comes from context or translation choice, not from a separate Croatian word.

Croatian word order is more flexible than English word order because cases show grammatical roles.

Starting with U vrtu puts the location first. It feels natural if the speaker wants to set the scene first:

• U vrtu je bila mala žaba kraj lokve.
  = In the garden, there was a small frog by a puddle.

You could also say:

• Mala žaba je bila u vrtu kraj lokve.

That is also grammatical, but the emphasis is a bit different. The original sentence sounds like a scene-setting description.

Je is the 3rd person singular present form of biti (to be) used as an auxiliary in the past tense.

Croatian often forms the past tense with:

• a present-tense auxiliary of biti
• plus a past participle

So:

• je bila = was (for feminine singular)

This is different from English, where was is just one word. In Croatian, it is built from two parts.

Ž is pronounced like the s in measure or the g in genre.

So žaba sounds roughly like:

• ZHA-ba

with the first sound like zh in English pronunciation guides.

Also:

• j in Croatian is like English y
• v is like English v
• lj, nj, č, ć, š, and ž are all important Croatian spelling/pronunciation patterns worth learning early

Yes, lokva is a normal noun meaning puddle.

It is a feminine noun, and like many feminine nouns, it ends in -a in the nominative singular.

Its forms include:

• nominative: lokva
• genitive: lokve

That is why after kraj you see lokve.

Yes. That is a very natural way to understand it.

Croatian often uses word order and the verb to be in ways that overlap with English there was / there were sentences.

So U vrtu je bila mala žaba kraj lokve can naturally correspond to:

• There was a small frog in the garden by the puddle
• A small frog was in the garden by the puddle

Both are possible, depending on context and translation style.