На кусте появился бутон.

Because the bud is understood as growing on the bush, attached to it. In Russian, на is natural for something located on the surface of or attached to a plant: на дереве, на кусте.

By contrast, в кусте or more often в кустах means in the bush / inside the bushes, among the branches or leaves.

So:

• На кусте появился бутон = a bud appeared on the bush
• В кустах = in the bushes

Because after на when it means location, Russian uses the prepositional case.

The noun changes like this:

• куст = bush
• на кусте = on the bush

So кусте is the prepositional singular form of куст.

Бутон is in the nominative singular because it is the subject of the sentence — the thing that appeared.

You can see this from the verb too: появился is masculine singular, and it agrees with бутон, which is a masculine singular noun.

In the Russian past tense, the verb agrees with the subject in gender and number.

Here the subject is бутон, which is:

• masculine
• singular

So the verb is:

• появился = masculine singular past

Compare:

• появился бутон = a bud appeared
• появилась роза = a rose appeared
• появились бутоны = buds appeared

Here -ся is just part of the verb появиться, which means to appear. It does not mean the bud is doing something to itself.

Many Russian verbs with -ся are best learned as complete dictionary forms. So it is helpful to memorize:

• появиться = to appear
• появляться = to appear, be appearing

In this sentence, появился simply means appeared.

It is perfective.

That means it presents the action as a single completed event: the bud appeared.

So На кусте появился бутон focuses on the moment or fact that a new bud came into being.

The imperfective partner is появляться:

• появлялся

That would usually suggest repeated appearance, process, or background description, not a single new event.

Usually, no — not if you mean a bud appeared as one new event.

• появился = a bud appeared, came out, showed up
• появлялся = was appearing / used to appear / would appear

So for a simple completed event, появился is the normal choice.

This is an important difference:

• На кусте появился бутон = a bud appeared on the bush
  
  • this describes a change or new development
• На кусте был бутон = there was a bud on the bush
  
  • this just states its existence in the past

So появился is more dynamic: something new came into view or came into existence.

Russian word order is flexible, and it often reflects emphasis rather than strict grammatical rules.

This order is natural because:

• На кусте sets the scene first: on the bush
• появился gives the event: appeared
• бутон comes last as the new information

So the sentence feels a bit like:

On the bush, a bud appeared.

You could also say:

• Бутон появился на кусте

That is also grammatical, but the emphasis feels a little different.

Russian has no articles, so бутон can mean either a bud or the bud, depending on context.

Out of context, a bud is usually the best translation here, because the sentence is introducing something new.

So:

• На кусте появился бутон most naturally = A bud appeared on the bush

But in the right context, English could also use the bud.

The stress is:

На кусте́ появи́лся буто́н.

Stressed syllables:

• кусте́
• появи́лся
• буто́н

A rough pronunciation guide:

na koos-TYE pa-ya-VEEL-sya boo-TON

The exact Russian sounds are not the same as English ones, but the stress pattern above is the key thing to learn first.

Sometimes, yes. Russian has a special locative form in -у with some masculine nouns after в and на.

With куст, you may hear на кусту, especially in expressions about something still growing on the bush, for example berries or tomatoes.

But на кусте is a normal, standard form and works perfectly well in this sentence.

So for a learner, the safest choice here is simply:

На кусте появился бутон.