Вчера в магазине не было свежей сельди, поэтому мы купили рыбу для супа.

Because this is an existential sentence: В магазине не было... means There was not / There wasn’t ... in the store.

In Russian, when you say that something was or wasn’t present somewhere, the past tense of быть is often used in the neuter singular form: было.

So:

• В магазине не было свежей сельди = There was no fresh herring in the store

Even though сельдь is feminine, Russian does not normally make быть agree with it in this kind of there was / there wasn’t construction.

Because after не было in this kind of negative existential sentence, Russian usually uses the genitive case.

So the dictionary form:

• свежая сельдь = fresh herring

changes to:

• свежей сельди = of fresh herring / any fresh herring

Both words go into the genitive singular:

• свежая → свежей
• сельдь → сельди

This is a very common pattern:

• На столе не было хлеба = There was no bread on the table
• В магазине не было молока = There was no milk in the store

Because в meaning in + location usually takes the prepositional case.

The noun is:

• магазин = store

In the prepositional singular:

• в магазине = in the store

This answers the question where?

Compare:

• в магазин = into the store (motion, accusative)
• в магазине = in the store (location, prepositional)

Поэтому means therefore, that’s why, or simply so.

It connects the two parts logically:

• there was no fresh herring in the store,
• so / therefore we bought fish for soup.

It is very common in everyday Russian and often appears near the beginning of the second clause.

Because купили is the perfective past tense of купить, and it presents the action as completed:

• купили = bought

Here the sentence refers to a specific completed purchase, so perfective is natural.

Compare:

• мы купили рыбу = we bought fish
• мы покупали рыбу = we were buying / used to buy fish / bought fish on some occasion with focus on process rather than result

In this sentence, the result matters: they ended up buying fish.

Because рыба is the direct object of купили.

The verb купить takes a direct object in the accusative case:

• рыба → рыбу

So:

• купили рыбу = bought fish

Since рыба is feminine singular, the accusative form is different from the nominative.

Because the preposition для always takes the genitive case.

So:

• суп = soup
• для супа = for soup

This phrase explains the purpose of the fish:

• рыбу для супа = fish for soup

Other examples:

• для детей = for children
• для дома = for the house/home
• для работы = for work

They are related, but not the same word.

• сельдь = herring
• рыба = fish

So the first part is specific: there was no fresh herring. The second part is more general: they bought fish for soup.

In English we might also say:

• There was no fresh herring, so we bought fish for soup.

So the sentence moves from a specific kind of fish to a general category.

Сельдь is a feminine noun even though it ends in -ь.

In Russian, many feminine nouns end in a soft sign:

• дверь = door
• ночь = night
• тетрадь = notebook
• сельдь = herring

Its forms include:

• nominative singular: сельдь
• genitive singular: сельди
• accusative singular: сельдь (same as nominative, because it is inanimate)

So in this sentence, after не было, it becomes genitive:

• сельди

Russian word order is flexible, and вчера is often placed first to set the time frame right away.

So:

• Вчера в магазине не было свежей сельди...

sounds natural and means something like:

• Yesterday, in the store, there was no fresh herring...

You could move вчера to another position, but the beginning is very common when time is the setting for the whole sentence.

Word-for-word, yes, but natural English would be:

• Yesterday there was no fresh herring in the store or
• Yesterday the store didn’t have any fresh herring

Russian often uses не было where English prefers there was no or didn’t have.

So it is best to understand:

• В магазине не было... = There was no ... in the store

rather than translating each word too literally.

Yes, absolutely. It is a very useful pattern.

A good chunk to remember is:

• Где-то не было чего-то = There was no something somewhere

Examples:

• В холодильнике не было молока = There was no milk in the fridge
• На столе не было книги = There was no book on the table
• В городе не было дождя = There was no rain in the city

This pattern helps you express absence naturally in Russian.

Because поэтому connects two clauses:

• Вчера в магазине не было свежей сельди
• поэтому мы купили рыбу для супа

In Russian, a comma is normally used before a linking word like поэтому when it joins two parts of a sentence in this way.

So the comma marks the relationship:

• first situation,
• then consequence.

No. After не было in this meaning, you need the genitive, not the accusative.

So:

• correct: не было свежей сельди
• incorrect: не было свежую сельдь

The accusative would be used if the noun were a direct object of a verb, but here it is part of the existential there was no... structure.

Here are the main forms:

• вчера = yesterday
• магазин = store, shop
• быть = to be
• свежий / свежая / свежее = fresh
• сельдь = herring
• поэтому = therefore, so
• мы = we
• купить = to buy
• рыба = fish
• для = for
• суп = soup

And the forms used in the sentence are:

• в магазине = prepositional singular
• было = past neuter singular of быть
• свежей сельди = genitive singular
• купили = past plural of купить
• рыбу = accusative singular
• для супа = genitive singular after для