На завтрак дедушка любит бублик с маслом.

Because на + accusative is a very common Russian pattern for for a meal / as a meal.

So:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner

In this sentence, На завтрак sets the context: as for breakfast / for breakfast.

It does not mean literal movement onto breakfast here. This is just an idiomatic use of на.

Because the preposition на often takes the accusative when it expresses destination, purpose, or something chosen for an occasion.

The base form is завтрак.
The accusative singular is also завтрак, so the form does not visibly change.

This is normal for many masculine inanimate nouns:

• nominative: завтрак
• accusative: завтрак

So even though the case is accusative, it looks the same as the dictionary form.

Russian word order is much freer than English word order.

The subject here is дедушка because it is in the nominative case and because любит agrees with it:

• дедушка = nominative singular
• любит = he loves/likes

So дедушка is the person doing the liking.

Putting На завтрак first is natural because it gives the setting first: For breakfast, grandpa likes...

English usually depends more heavily on word order. Russian depends more on case endings and verb agreement.

Любит is the 3rd person singular present tense form of любить.

Conjugation of любить in the present:

• я люблю
• ты любишь
• он/она любит
• мы любим
• вы любите
• они любят

Since дедушка is he, the verb is любит.

In sentences like this, любить often means to like in the sense of habit or preference, not only strong emotional love.

So here it means something like Grandpa likes to have...

Yes, it is accusative.

But бублик is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is usually the same as the nominative singular.

So:

• nominative: бублик
• accusative: бублик

That is why the form does not change.

Compare that with an animate masculine noun, where accusative usually matches the genitive:

• nominative: кот
• accusative: кота

But бублик is inanimate, so no visible change.

Because the preposition с meaning with normally takes the instrumental case.

The noun масло is neuter:

• nominative: масло
• instrumental: маслом

So:

• с маслом = with butter

This is one of the most important uses of the instrumental case in Russian: after с when it means with.

Here с маслом describes the kind of бублик: a bagel/ring-shaped bread with butter.

It works much like English with butter in food descriptions:

• хлеб с сыром = bread with cheese
• картошка с мясом = potatoes with meat
• бублик с маслом = a bagel with butter

So the phrase does not mean that the butter is doing something instrumentally in a literal sense. It is just the normal Russian grammar after с.

Because Russian has no articles.

So бублик can mean:

• a bagel
• the bagel
• sometimes just bagel in a general sense

The exact meaning depends on context.

In this sentence, English might translate it as a bagel with butter or bagels with butter, depending on how natural the translation is in context. Russian does not force you to choose with an article.

Yes. That is also grammatical.

Russian word order can shift for emphasis, topic, or style.

Compare:

• На завтрак дедушка любит бублик с маслом.
  Focuses first on for breakfast.

• Дедушка любит на завтрак бублик с маслом.
  Starts with grandpa as the topic.

Both are natural. The basic meaning stays the same, but the information flow changes a little.

It can, but it is not necessary.

Russian often uses любить + noun directly when talking about food preferences:

• Я люблю кофе.
• Он любит суп.
• Дедушка любит бублик с маслом.

This already means that the person likes having/eating that food.

If you say любит есть, you emphasize the action of eating a little more:

• Дедушка любит есть бублик с маслом.

That is also correct, but slightly heavier and less compact than the original sentence.

Yes.

Дедушка refers to a male person, so it is grammatically masculine, even though it has an ending that looks like many feminine nouns.

This is a common pattern with some nouns for male people, especially familiar or affectionate words:

• дедушка = grandpa
• папа = dad
• дядя = uncle

They decline partly like -а / -я nouns, but they are still treated as masculine in agreement:

• мой дедушка
• добрый дедушка
• дедушка любит

So the ending alone does not always tell you the gender.

In most cases, it is closest to for breakfast.

It usually refers to what someone has, eats, makes, or wants as breakfast:

• Я ем кашу на завтрак. = I eat porridge for breakfast.
• Что у нас на завтрак? = What are we having for breakfast?

If you want to talk about something happening during breakfast, Russian would often use a different construction, depending on the context.

So in this sentence, на завтрак means the food preference for that meal, not simply while at breakfast.