Na travi je malo bijelog perja, a iznad vode leti cijeli roj pčela.

Because na means on, and here it expresses a fixed location: on the grass.

When na means location, it normally takes the locative case.
So:

• trava = grass
• na travi = on the grass

If motion were involved, Croatian might use a different case. But here nothing is moving onto the grass; the feathers are simply located there.

Here je is the 3rd person singular present form of biti = to be.

In this sentence, it works like English there is / there are:

• Na travi je malo bijelog perja = There is a little white feather/down on the grass

Croatian often uses biti in existential sentences like this, where English uses there is/there are.

Even though English says there is, Croatian just uses is:

• je = is

Because malo as a quantity word usually requires the genitive.

So after malo = a little / not much, the following noun goes into the genitive:

• perje = feathering, plumage, down, feathers as a mass
• perja = genitive singular

And the adjective must match that form:

• bijelo perje = white feathers/down
• bijelog perja = of white feathers/down

So:

• malo bijelog perja = a little white feather/down

This is very common in Croatian:

• puno vode = a lot of water
• malo kruha = a little bread
• mnogo ljudi = many people

Because perje is often treated as a mass/collective noun in Croatian.

So instead of thinking of separate individual feathers, Croatian can present it as a substance or collection:

• perje = feathering / feathers / down

After malo, it becomes:

• malo perja = a little feather/down, a small amount of feathers

English and Croatian do not always package this idea the same way. English often prefers a plural noun, but Croatian can naturally use this collective singular noun.

Because the adjective has to agree with perja, which is in the genitive singular.

Base form:

• bijel = white

Here it changes to:

• bijelog = genitive singular form

So:

• bijelo perje = white feathering/down
• bijelog perja = of white feathering/down

The ending is not random; it shows case, gender, and number agreement with the noun.

Because iznad means above and it takes the genitive case.

So:

• voda = water
• vode = genitive singular

That gives:

• iznad vode = above the water

This is worth memorizing as a pattern:

• iznad + genitive

Similar examples:

• iznad grada = above the city
• iznad kuće = above the house

Because the grammatical subject is roj, not pčela.

• roj = swarm
• pčela = of bees

So the phrase cijeli roj pčela literally means a whole swarm of bees.

Since roj is singular, the verb is singular too:

• leti = flies

This is similar to English:

• A swarm of bees is flying
  not
• A swarm of bees are flying (though some speakers may say this informally)

In Croatian, the agreement with roj is the normal choice here.

Because after roj Croatian uses the genitive plural.

• pčela = bee
• pčele = bees (nominative plural)
• pčela = of bees (genitive plural)

So:

• roj pčela = a swarm of bees

This is a very common structure with quantity or group nouns:

• čaša vode = a glass of water
• skupina ljudi = a group of people
• roj pčela = a swarm of bees

Cijeli means whole, entire, or sometimes full depending on context.

So:

• roj = swarm
• cijeli roj = a whole swarm

In this sentence it emphasizes that it is not just a few bees, but an entire swarm.

The adjective cijeli agrees with roj:

• masculine singular nominative: cijeli

because roj is masculine singular.

A is a conjunction. Here it links two clauses and gives a slight sense of contrast or scene-shift.

So it can feel like:

• and
• while
• whereas

depending on context.

In this sentence:

• Na travi je malo bijelog perja, a iznad vode leti cijeli roj pčela.

it sounds like:

• There is a little white feather/down on the grass, and above the water a whole swarm of bees is flying.

It is not a strong contradiction; it is more like moving the listener’s attention from one image to another.

Yes, Croatian word order is flexible, and this order is natural because it foregrounds the locations:

• Na travi = on the grass
• iznad vode = above the water

So the sentence first sets the scene, then tells what is there or what is happening.

A more neutral English-like order might be possible, but the original sounds very natural in Croatian because it presents the location first:

• Na travi je...
• iznad vode leti...

This is common in descriptions.

For example, Croatian often likes:

• Na stolu je knjiga. = There is a book on the table. rather than always starting with the noun.

Yes, leti is present tense of letjeti = to fly.

In Croatian, the present tense can often cover both:

• flies
• is flying

depending on context.

Here, because the sentence describes a scene in progress, English would most naturally say:

• a whole swarm of bees is flying above the water

But a more literal translation with flies is also possible.

Often yes, but not always mechanically.

Perje can mean:

• feathers
• plumage
• down

In this sentence, malo bijelog perja suggests a small amount of white feather matter lying on the grass. Depending on context, English could say:

• a little white feather
• a few white feathers
• a little white down
• a bit of white plumage

So feathers is fine, but Croatian is slightly more collective here than English often is.

Yes, but it would mean something slightly different in focus.

• Pčele lete iznad vode. = Bees are flying above the water.
• Iznad vode leti cijeli roj pčela. = Above the water, a whole swarm of bees is flying.

The original highlights the image of a swarm as one moving mass.
Using pčele lete would focus more on the bees as separate creatures.

So the original wording is more vivid and specific.