Утром я съела одну ватрушку и выпила кефир.

Утром means in the morning.

Grammatically, it is the instrumental singular of утро, but in sentences like this it works as an adverbial time expression. Russian often uses these forms without a preposition:

• утром = in the morning
• днём = in the daytime / during the day
• вечером = in the evening
• ночью = at night

So Утром я... is the normal way to say In the morning, I....
В утро is not the normal choice for this meaning.

Because the speaker is female.

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

• я съел / выпил = I ate / drank — if the speaker is male
• я съела / выпила = I ate / drank — if the speaker is female

So this sentence tells you that the speaker is a woman or girl.

The ъ in съела is a hard sign. It does not have its own sound, but it separates the prefix from the following vowel.

Here the verb is:

• с-
  • ела

Because a prefix ending in a consonant comes before е, Russian writes ъ:

• съела
• compare similar patterns like объём, подъезд

Its job is to show that the е is pronounced like ye, not just a softened e after the consonant.

Because съела and выпила are perfective verbs, and they show completed actions.

This sentence describes one finished event: the speaker ate the pastry and drank the kefir.

• съела, выпила = ate up, drank up / finished
• ела, пила = was eating, was drinking, used to eat/drink, or simply ate/drank without focusing on completion

So in a one-time breakfast statement, the perfective forms are very natural.

In this sentence, the most important thing is that they are part of the perfective partner of the verb:

• есть → съесть
• пить → выпить

These perfective verbs usually suggest that the food or drink was consumed as a completed action.

It is usually better for learners to memorize them as verb pairs, rather than trying to translate the prefixes literally every time.

Because ватрушку is the direct object of съела, so it must be in the accusative case.

The basic form is:

• одна ватрушка = one vatrushka

But after съела, it becomes:

• одну ватрушку

Both words change because один agrees with the noun:

• одна → одну
• ватрушка → ватрушку

Also, after one, the noun stays singular in Russian.

It is also a direct object, so it is in the accusative case too — but for an inanimate masculine noun, the accusative singular is usually the same as the nominative.

So:

• nominative: кефир
• accusative: кефир

That is why it looks unchanged.

Because Russian has no articles.

Russian does not have words equivalent to English a/an and the as a normal grammar feature. Context shows whether something is definite or indefinite.

So:

• одну ватрушку = one vatrushka
• кефир = kefir / the kefir / some kefir

The exact nuance depends on context.

Russian often allows a mass noun like кефир to stand by itself.

So выпила кефир is perfectly possible and natural. It means the speaker drank kefir, with the exact amount left to context.

If you want to be more specific, you can add a measure:

• выпила стакан кефира = drank a glass of kefir
• выпила бутылку кефира = drank a bottle of kefir

But the original sentence is fine as it is.

Russian can drop subject pronouns, but here я is very useful.

A key point: in the past tense, Russian verbs do not show person clearly.
For example, съела could mean:

• I ate — if the speaker is female
• she ate

So я helps identify the subject.

That is one reason pronouns are often kept more readily in the past tense than beginners sometimes expect.

Yes, Я утром съела одну ватрушку и выпила кефир is also possible.

Russian word order is more flexible than English word order. The version in your sentence starts with Утром to set the time frame first:

• Утром я... = As for the morning / In the morning, I...

This is very natural and neutral. Moving words around usually changes emphasis, not the core meaning.

Yes, sometimes — but it adds a different nuance.

• выпила кефир = neutral direct object, the default choice
• выпила кефира = genitive with a partitive sense, often meaning drank some kefir

So кефира can sound more like an indefinite amount.
The sentence you have uses кефир, which is the most straightforward neutral form.