В магазине я купила брынзу и тонкую салями для бутербродов.

Купила is the past tense feminine singular form of купить (to buy).

In Russian past tense, the verb agrees with the speaker or subject in gender and number:

• я купил = I bought (said by a man)
• я купила = I bought (said by a woman)
• мы купили = we bought

So this sentence tells us the speaker is female.

В магазине is in the prepositional case.

Here, в means in / at, referring to a location:

• в магазине = in the store / at the store

After в:

• if you are talking about location, you usually use the prepositional case
• if you are talking about motion into somewhere, you usually use the accusative

Compare:

• Я была в магазине. = I was at the store.
• Я пошла в магазин. = I went to the store.

So in your sentence, the action happened at the store, not into the store, which is why it is в магазине.

Because брынзу is a direct object, and here it is in the accusative case.

The noun брынза is feminine, and feminine nouns ending in -а usually change like this in the accusative singular:

• nominative: брынза
• accusative: брынзу

This is very common:

• вода → воду
• машина → машину
• колбаса → колбасу

Since the speaker bought the cheese, it is the thing directly affected by the action, so Russian uses the accusative.

Because салями is also a direct object, so the adjective has to agree with it in the accusative singular feminine.

The adjective тонкая changes like this:

• nominative feminine: тонкая
• accusative feminine: тонкую

So:

• тонкая салями = nominative
• тонкую салями = accusative

The interesting part is that салями itself usually does not change. It is an indeclinable noun in Russian, but it is treated as feminine. So the adjective changes, even though the noun stays the same.

Because салями is an indeclinable noun.

Some Russian nouns, especially many borrowed words, do not change across cases. Instead, other words around them show the grammar. Here the adjective tells you what case the noun is functioning in:

• тонкая салями
• тонкую салями
• о тонкой салями

The noun салями stays the same, but the adjective changes.

Because для (for) requires the genitive case.

The noun бутерброд becomes бутербродов, which is the genitive plural form:

• nominative singular: бутерброд
• nominative plural: бутерброды
• genitive plural: бутербродов

So:

• для бутербродов = for sandwiches

This is a very useful rule to remember:

• для + genitive

Examples:

• для друга = for a friend
• для детей = for children
• для дома = for the house/home

Because after для, Russian needs the genitive, not the nominative.

• бутерброды is the nominative plural form: sandwiches
• бутербродов is the genitive plural form: of sandwiches / for sandwiches after для

So the phrase must be:

• для бутербродов

not

• для бутерброды

Russian word order is more flexible than English word order because grammatical roles are shown by case endings, not mainly by position.

So both of these are possible:

• В магазине я купила брынзу...
• Я купила брынзу... в магазине.

The difference is mainly about focus or what the speaker wants to highlight.

Starting with В магазине makes the setting come first, something like:

• At the store, I bought...

This is very natural in Russian. It often sounds like the speaker is setting the scene before giving the main information.

Yes, often it could.

Russian often leaves out subject pronouns when they are clear from the verb form and context. Since купила already shows first person singular and feminine, you can say:

• В магазине купила брынзу и тонкую салями для бутербродов.

That can sound natural in context.

However, я is often included:

• for clarity
• for emphasis
• when introducing a new subject
• when contrasting with someone else

So both are possible, but я is perfectly normal here.

Yes. They are two direct objects joined by и (and):

• брынзу
• тонкую салями

Both are things the speaker bought, so both are in the accusative.

The difference is just that they behave differently:

• брынза changes form: брынза → брынзу
• салями stays the same because it is indeclinable
• the adjective тонкую shows the accusative clearly

Купила is from купить, which is perfective.

Perfective verbs in Russian usually present an action as:

• completed
• whole
• result-focused

So я купила means I bought in the sense of I successfully bought it / I ended up buying it.

The imperfective partner is покупать:

• я покупала can mean I was buying / I used to buy / I bought with focus on process or repeated action, depending on context

In this sentence, the speaker is talking about a completed shopping action, so купила is the natural choice.

Grammatically, it literally means thin salami.

In context, that could mean:

• a thin type of salami
• a narrow salami sausage
• sometimes learners may interpret it as salami intended to be sliced thinly, but the adjective itself directly means thin

If Russian wanted to stress thin slices, it would usually be expressed more explicitly in another way.

So the key grammar point here is simply that тонкую is an adjective agreeing with салями.

Because Russian does not have articles like English a/an and the.

So a sentence like this can mean:

• I bought brynza and a thin salami for sandwiches
• I bought the brynza and the thin salami for sandwiches

Which interpretation is intended depends on context.

Russian usually expresses definiteness through:

• context
• word order
• stress
• sometimes demonstratives like этот / эта / это if needed

So the absence of articles here is completely normal.