Моя тётя любит сельдь с картошкой, а я ем её только по праздникам.

Because моя has to agree with тётя in gender, number, and case.

• тётя is feminine
• it is singular
• here it is in the nominative case because it is the subject

So the correct form is:

• мой = masculine
• моя = feminine
• моё = neuter
• мои = plural

That is why Russian uses моя тётя.

It is in the accusative case because it is the direct object of любит.

However, for many inanimate feminine nouns like сельдь, the accusative singular is the same as the nominative singular. So:

• nominative: сельдь
• accusative: сельдь

That is why the form does not change visibly here.

Nouns ending in -ь can be either masculine or feminine, so you often have to learn their gender individually.

Сельдь is a feminine noun. You can tell from dictionary information or from agreement in other forms, for example:

• эта сельдь
• вкусная сельдь

So the soft sign by itself does not automatically tell you the gender.

Because the preposition с meaning with usually takes the instrumental case when it shows accompaniment.

So:

• dictionary form: картошка
• instrumental singular: картошкой

This is a very common pattern:

• чай с лимоном
• хлеб с маслом
• рыба с картошкой

Here с картошкой means with potatoes.

А often marks a contrast or a comparison of topics.

In this sentence:

• Моя тётя любит...
• а я ем...

the speaker is setting my aunt and me side by side: my aunt likes it, whereas I only eat it on holidays.

Very roughly:

• и = and
• но = but
• а = and/but/whereas, depending on context

Here а feels very natural because it contrasts two people’s habits without sounding like a strong contradiction.

Because есть is the infinitive, while ем is the 1st person singular present-tense form: I eat.

The verb есть is irregular:

• я ем
• ты ешь
• он/она ест
• мы едим
• вы едите
• они едят

So я ем means I eat.

Её is the accusative singular feminine pronoun, meaning her/it depending on context.

Here it means it and refers to сельдь, which is:

• feminine
• singular
• the object of ем

So the grammar is:

• noun replaced: сельдь
• pronoun form needed: её

Russian often uses pronouns this way instead of repeating the noun.

Grammatically, it could, because both сельдь and картошка are feminine singular nouns.

But in context, её is understood as referring to сельдь.

Why?

• сельдь is the main food being discussed
• с картошкой is just an added phrase meaning with potatoes
• the sentence contrast is really about the aunt liking herring and the speaker eating it only on holidays

So the natural interpretation is I eat herring only on holidays.

По праздникам means on holidays, on festive occasions, or during holiday times in a habitual sense.

The preposition по is used in some time expressions with the dative plural. So:

• dictionary form: праздник
• dative plural: праздникам

This construction often describes something that happens regularly on those occasions, not just once.

Compare:

• по праздникам = on holidays, whenever holidays come
• not a single specific holiday, but a recurring situation

Because the sentence describes general habits and usual behavior.

• любит = likes, generally likes
• ем = eat, in the sense of I eat it sometimes / as a habit

Russian normally uses the imperfective for repeated, habitual, or ongoing situations.

A perfective verb would suggest a single completed action, which would not fit this sentence well.

Because только is limiting the time expression: only on holidays.

So:

• я ем её только по праздникам = I eat it only on holidays

If you moved только, the meaning could change:

• я ем только её = I eat only it
• только я ем её по праздникам = only I eat it on holidays

Russian word order is flexible, so the position of только matters a lot for emphasis and meaning.