На завтрак сестра намазала авокадо на хлеб и добавила сверху кусочек сыра.

На завтрак is an idiomatic Russian way to mean for breakfast.

• на
  • accusative is often used with meals:
    • на завтрак = for breakfast
    • на обед = for lunch
    • на ужин = for dinner

So here it sets the time/occasion: For breakfast, ...

It does not sound like physical on breakfast in Russian.

Russian has no articles, so there is no direct equivalent of a or the.

Also, Russian often leaves out possessives like my when the context makes them obvious. So сестра could mean:

• the sister
• my sister
• his/her sister

depending on context.

In a natural translation, English would usually say my sister or his/her sister, but Russian often does not need to specify that.

In the past tense, Russian verbs agree with the subject in gender and number.

The infinitive is намазать. In the past tense:

• намазал = he spread
• намазала = she spread
• намазало = it spread
• намазали = they spread

Since сестра is feminine, the verb is намазала.

The same thing happens with добавила:

• добавила = she added

Авокадо is an indeclinable noun in Russian, so its form does not change across cases.

In this sentence, it is the direct object of намазала, so logically it is in the accusative, but the form stays авокадо.

So:

• nominative: авокадо
• accusative: авокадо
• genitive: авокадо

and so on.

This is common with some borrowed words in Russian.

Here на хлеб means onto the bread / on the bread.

After на, Russian can use different cases depending on the meaning:

• на + accusative = movement/placement onto something
• на + prepositional = location on something

Compare:

• намазала авокадо на хлеб = spread avocado onto the bread
• авокадо на хлебе = the avocado is on the bread

In your sentence, something is being put onto the bread, so Russian uses на + accusative:

• хлеб is accusative here

For an inanimate masculine noun like хлеб, nominative and accusative look the same, so you just see хлеб.

Yes, Russian can use two different patterns with verbs like мазать / намазать.

1. Spread something onto something

   • намазать авокадо на хлеб
   • direct object = the thing being spread

2. Spread something with something

   • намазать хлеб авокадо
   • direct object = the surface being covered

So both patterns are possible, but they highlight slightly different things.

In your sentence:

• намазала авокадо на хлеб focuses on putting the avocado onto the bread

A very similar contrast exists with butter:

• намазать масло на хлеб = spread butter on bread
• намазать хлеб маслом = spread the bread with butter

Намазала and добавила are perfective past forms. Perfective verbs usually present the action as completed.

Here the sentence describes a finished sequence of actions:

• she spread the avocado
• she added a piece of cheese

That is why perfective verbs are natural.

Compare:

• мазала / добавляла = was spreading / used to spread / spread repeatedly
• намазала / добавила = spread / added, as completed actions

So this sentence sounds like a single completed breakfast-preparation event.

No. In намазала, the на- is a prefix attached to the verb мазать.

• мазать = to smear, spread
• намазать = to spread onto, smear on

This на- is part of the verb itself and is not the separate word нам.

So:

• нам = to us
• намазала = she spread / she smeared on

They just happen to look similar at the beginning.

Сверху means on top, over the top, or from above, depending on context.

In this sentence:

• добавила сверху кусочек сыра = added a piece of cheese on top

It tells you where the cheese was added: on top of the avocado/bread.

It is an adverb, so it does not need a noun after it here. Russian often uses сверху by itself when the object is understood from context.

After words meaning a piece of, a bit of, a glass of, a cup of, Russian normally uses the genitive for the substance or whole.

So:

• кусочек = a little piece
• сыра = of cheese

This is the same pattern as:

• кусок хлеба = a piece of bread
• чашка чая = a cup of tea
• бутылка воды = a bottle of water

So кусочек сыра literally means a little piece of cheese.

Кусочек is a diminutive form of кусок.

• кусок = a piece, a chunk
• кусочек = a little piece, a small piece

Using кусочек often sounds more natural, softer, or more specific when talking about food, especially a small amount.

So:

• кусок сыра = a piece/chunk of cheese
• кусочек сыра = a small piece of cheese

Russian word order is fairly flexible, and this sentence could be rearranged without changing the core meaning.

The given order:

• На завтрак сестра намазала авокадо на хлеб и добавила сверху кусочек сыра.

This sounds natural and starts with the time/context: For breakfast.

Other possible orders include:

• Сестра на завтрак намазала авокадо на хлеб и добавила сверху кусочек сыра.
• Сестра намазала авокадо на хлеб и сверху добавила кусочек сыра.

The main differences are in focus and style, not basic meaning.

Russian often moves elements around to emphasize:

• time
• topic
• contrast
• new information

Grammatically, авокадо just means avocado. Russian does not explicitly tell you from the noun form alone whether it is:

• sliced avocado
• mashed avocado
• avocado spread

The verb намазала strongly suggests something spreadable, so in context a native speaker would usually imagine mashed avocado or some kind of avocado spread/paste.

So the exact texture comes more from the verb and the real-world context than from the noun itself.