На завтрак я пью яблочный сок.

In Russian, на + accusative is often used to mean for in the sense of food, meals, or purpose.

So:

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

Literally, it may feel like onto breakfast or for breakfast, but the natural English translation is simply for breakfast or at breakfast depending on context.

In this sentence, На завтрак я пью яблочный сок means that apple juice is what the speaker drinks as their breakfast drink.

Because на here takes the accusative case, and завтрак is a masculine inanimate noun.

For many masculine inanimate nouns, the accusative singular looks exactly the same as the nominative singular:

• nominative: завтрак
• accusative: завтрак

So the form does change grammatically, but it just happens to look identical.

Yes, я could be omitted.

Russian often leaves out subject pronouns when the verb ending already makes the subject clear. Since пью clearly means I drink, you can say:

• На завтрак пью яблочный сок.

This still means I drink apple juice for breakfast.

Including я is not wrong at all. It can make the sentence clearer, more neutral, or slightly more emphatic.

Пью means I drink.

Its dictionary form is пить = to drink.

This verb is a little irregular in the present tense:

• я пью = I drink
• ты пьёшь = you drink
• он/она пьёт = he/she drinks
• мы пьём = we drink
• вы пьёте = you drink
• они пьют = they drink

A learner may notice that пить does not look very similar to пью at first, so it is a good verb form to memorize.

The verb пить is imperfective, and in the present tense it usually describes:

• a habitual action: I drink apple juice for breakfast
• a general fact
• something happening now, depending on context

In this sentence, without extra context, it most naturally sounds like a habit or usual routine: I drink apple juice for breakfast.

If you wanted to emphasize a completed one-time action, Russian would often use a perfective verb in the past or future, not the present, because perfective verbs do not normally have a true present tense in Russian.

Russian usually uses an adjective + noun structure where English often uses a noun as an adjective.

So:

• яблоко = apple
• яблочный = apple / made from apples
• сок = juice

Therefore:

• яблочный сок = apple juice

You cannot normally put two nouns together the way English does in apple juice. Russian prefers the adjective form.

Because it has to agree with сок.

Russian adjectives change to match the noun in:

• gender
• number
• case

Here, сок is:

• masculine
• singular
• accusative

Since сок is masculine singular, the adjective also takes the masculine singular form:

• яблочный сок

Because сок is inanimate, its accusative form looks like the nominative, so the adjective also looks like the nominative masculine form here.

Compare:

• яблочный сок = apple juice
• яблочная пастила = apple fruit candy/paste
• яблочное пюре = apple purée

Yes, it is in the accusative case, because it is the direct object of пью.

However, сок is a masculine inanimate noun, and for masculine inanimate nouns, the accusative singular is the same as the nominative singular:

• nominative: сок
• accusative: сок

So the word is in the right case; it just does not visibly change.

If it were an animate masculine noun, the accusative would look different.

Yes. Russian word order is much more flexible than English word order.

This sentence:

• На завтрак я пью яблочный сок.

could also appear as:

• Я пью яблочный сок на завтрак.
• Я на завтрак пью яблочный сок.
• Яблочный сок я пью на завтрак.

These versions all mean roughly the same thing, but the emphasis changes:

• На завтрак first puts focus on for breakfast
• Я first makes the subject more prominent
• Яблочный сок first highlights apple juice

The original sentence is very natural.

Not exactly.

• на завтрак = for breakfast
• утром = in the morning

These are related ideas, but not the same.

For example:

• Утром я пью яблочный сок. = In the morning I drink apple juice.
• На завтрак я пью яблочный сок. = For breakfast I drink apple juice.

The first is about time of day. The second is about the meal.

Russian has no articles like a, an, or the.

So яблочный сок can mean:

• apple juice
• an apple juice
• the apple juice

The exact meaning depends on context.

In this sentence, English naturally uses no article too: I drink apple juice for breakfast. But in other contexts, Russian still would not use articles, even where English would need one.

A helpful stress guide is:

На зА́втрак я пью́ я́блочный сок.

Main stresses:

• зА́втрак
• пью́
• я́блочный
• сок (monosyllabic, so it is naturally stressed)

A rough pronunciation guide for an English speaker:

• На = nah
• зА́втрак = ZAV-trək
• я пью́ = ya pyu
• я́блочный = YA-blach-ny
• сок = soke, but without a strong final English k release

The cluster in пью can feel difficult at first. Try saying it as one syllable, not pee-yoo, but more like pyu.

Grammatically, you could build phrases like that in some contexts, but it is not the normal way to say apple juice.

The standard expression is:

• яблочный сок

Using сок из яблок would sound more literal, like juice made from apples, and it is less natural for an everyday sentence like this one.

So if you simply mean apple juice, stick with яблочный сок.