На цветке сидит маленький жук.

Because на here means on in the sense of location, so Russian uses the prepositional case:

• цветок = flower
• на цветке = on the flower

The noun changes because it is not the basic dictionary form anymore.

A useful contrast:

• на цветке = on the flower (location)
• на цветок = onto the flower (motion toward it, usually with the accusative)

So in this sentence, the beetle is already sitting there, so на цветке is correct.

Цветок is a masculine noun, and after на in a location meaning, it usually goes into the prepositional singular.

The pattern is:

• nominative: цветок
• prepositional: на цветке

This change is partly due to the noun ending and partly due to a spelling rule in Russian. Many masculine nouns ending in -ок change to -ке in this form:

• замок → в замке
• урок → на уроке
• цветок → на цветке

So you should learn на цветке as the normal prepositional form.

Yes. In Russian, сидеть is used much more broadly than English to sit.

It can describe a person sitting, but also an animal, bird, insect, or even sometimes an object that is positioned in place. So for a beetle on a flower, сидит sounds natural.

In English, you might say:

• A small beetle is sitting on the flower
• There is a small beetle on the flower

Russian often prefers сидит to describe the beetle’s position.

Russian usually does not use a present-tense verb meaning there is in simple existence sentences.

So instead of saying something like On the flower there is a small beetle, Russian often says something more like:

• На цветке сидит маленький жук
  literally: On the flower sits a small beetle

This is a very normal Russian way to introduce something in a place.

Both are possible, but they emphasize different things.

• На цветке сидит маленький жук puts the location first. It feels a bit like: On the flower, there is a small beetle sitting.
• Маленький жук сидит на цветке starts with the beetle itself: The small beetle is sitting on the flower.

Russian word order is more flexible than English word order. The sentence in your example is natural if the speaker wants to present the scene first and then mention what is there.

Because маленький describes жук, and adjectives in Russian must agree with the noun in:

• gender
• number
• case

Here, жук is:

• masculine
• singular
• nominative

So the adjective must also be masculine singular nominative:

• маленький жук = small beetle

Some other forms would be:

• маленькая книга = small book
• маленькое окно = small window
• маленькие жуки = small beetles

Because жук is the subject of the sentence.

Even though the sentence starts with На цветке, that phrase is just telling you the location. The thing that is actually doing the sitting is маленький жук, so it stays in the nominative case.

You can think of the structure like this:

• На цветке = where?
• сидит = is sitting
• маленький жук = who/what is sitting?

That is why жук is nominative.

Russian has no articles like a or the.

So На цветке сидит маленький жук does not explicitly tell you whether the speaker means:

• On a flower sits a small beetle
• On the flower sits a small beetle
• On the flower sits the small beetle

Context decides.

In many cases, English translations add a or the based on what sounds natural in the situation.

Not always. На has several common meanings depending on context, including:

• on
• at
• to
• sometimes other location-related meanings

Examples:

• на столе = on the table
• на работе = at work
• на концерт = to the concert

In your sentence, на цветке clearly means on the flower.

Grammatically, you might hear or understand it, but it is usually less natural in this context.

Russian often prefers a position verb such as:

• сидит = is sitting
• лежит = is lying
• стоит = is standing

So На цветке сидит маленький жук sounds more idiomatic because it describes the beetle’s position on the flower.

A rough pronunciation guide:

• На = na
• цветке = tsvee-TKYE
• сидит = see-DEET
• маленький = MA-len-kee
• жук = zhook

A few notes:

• ц sounds like ts
• ж sounds like the s in measure
• The stress is on:
  • цветкЕ
  • сидИт
  • мАленький
  • жук is one syllable

So the rhythm is roughly:

na tsvee-TKYE see-DEET MA-len-kee zhook

Here are the dictionary forms:

• на = preposition
• цветке → цветок = flower
• сидит → сидеть = to sit
• маленький = small
• жук = beetle

This is useful because Russian words often change form in actual sentences, especially nouns, adjectives, and verbs.