В магазине было столько людей, что я так и не примерила туфли.

Because this is the Russian existential pattern: В магазине было столько людей = There were so many people in the store.

In this structure, Russian often uses:

• a place expression: в магазине
• the verb быть in the neuter singular past: было
• the thing/person that existed, often in the genitive: людей

So even though людей refers to many people, the verb is commonly было, not были, in this kind of there was / there were sentence.

Compare:

• В комнате было много людей. = There were many people in the room.
• На улице было мало машин. = There were few cars on the street.

Because after столько (so many / that many), Russian uses the genitive plural.

• люди = nominative plural, people
• людей = genitive plural, of people

So:

• столько людей = so many people

This is very common with quantity words:

• много людей = many people
• мало времени = little time
• несколько книг = several books
• столько людей = so many people

This is a very common result structure:

• столько ..., что ... = so many ..., that ...
• more generally, so much / so many ..., that ...

In your sentence:

• В магазине было столько людей, что...
• There were so many people in the store, that...

The second clause gives the result of the first:

• there were so many people
• that I didn’t end up trying on the shoes

This works with other words too:

• Он был так устал, что сразу уснул. = He was so tired that he fell asleep right away.
• Было столько работы, что мы ушли поздно. = There was so much work that we left late.

Так и не is a very useful expression. It often means:

• never did
• didn’t end up
• still didn’t
• failed to, in the end

So:

• я так и не примерила туфли = I never did try on the shoes / I ended up not trying on the shoes

It adds a nuance that the action was expected, intended, or possible, but in the end it did not happen.

Examples:

• Я так и не позвонил ей. = I never did call her.
• Мы так и не нашли кафе. = We never ended up finding the café.
• Он так и не ответил. = He never replied.

So here it suggests: I probably intended to try them on, but because of the crowd, it didn’t happen.

Примерила is the perfective past form of примерить, while примеряла would be the imperfective past of примерять.

Here, the speaker is talking about whether the action happened at all. It did not happen. Russian often uses the perfective in negation when the meaning is I didn’t manage to do it / I never did it.

So:

• не примерила = did not try on (not even once; the action never happened)
• не примеряла would sound more like was not trying on / did not try on repeatedly / was not engaged in trying on

Because the sentence is about a single completed opportunity that failed to happen, примерила is the natural choice.

The phrase так и не especially fits well with perfective verbs:

• так и не сделал
• так и не увидела
• так и не примерила

Not exactly. Примерить means to try on clothes, shoes, or accessories.

So:

• примерить туфли = to try on shoes
• примерить платье = to try on a dress

This is different from:

• попробовать = to try, attempt
• пробовать = to try, taste, attempt

For clothing and shoes, Russian normally uses:

• мерить / примерять / примерить

Examples:

• Я хочу примерить это пальто. = I want to try on this coat.
• Она примеряла шляпы. = She was trying on hats.

Yes. In Russian, туфли is normally used in the plural when talking about a pair of shoes.

• туфля exists as a singular form, but usually means one shoe
• туфли = shoes / a pair of shoes

So:

• примерить туфли = try on the shoes / try on a pair of shoes

This is similar to English, where shoes is also often plural.

Other Russian nouns often used this way:

• ботинки = boots/shoes
• сапоги = boots
• джинсы = jeans
• брюки = trousers/pants

It is in the accusative here, but for inanimate plural nouns, the accusative is usually the same as the nominative.

So:

• nominative plural: туфли
• accusative plural: туфли

Because примерить takes a direct object, туфли is in the accusative.

Compare:

• Я вижу столы. = I see tables.
  (accusative plural = nominative plural, because столы are inanimate)

But with animate nouns:

• Я вижу людей. = I see people.
  (accusative plural matches genitive plural)

В магазине is in the prepositional case because it answers where?

• в магазине = in the store

With в, Russian commonly uses:

• prepositional for location: where?
• accusative for direction: to where?

Compare:

• Я в магазине. = I am in the store.
• Я иду в магазин. = I am going to the store.

So in your sentence, the meaning is location, not motion, which is why it is в магазине.

Russian often does not need a separate word for English there in existential sentences.

English says:

• There were so many people in the store

Russian simply says:

• В магазине было столько людей

Literally, it is closer to:

• In the store there-was so many people

But Russian usually expresses this idea with word order and structure, not with a separate dummy subject like English there.

So the sentence is completely normal without any extra word.

Yes, absolutely.

• В магазине было столько людей, что...
• В магазине было так много людей, что...

Both can mean:

• There were so many people in the store that...

A small nuance:

• столько людей often feels like that many / so many
• так много людей feels like so many / so much

In many contexts they are interchangeable.

Examples:

• Было столько шума, что я ничего не слышал.
• Было так много шума, что я ничего не слышал.

Both are natural.

The sentence is natural as written, but Russian word order is more flexible than English.

Your sentence:

• В магазине было столько людей, что я так и не примерила туфли.

This order is good because it starts with the setting (in the store) and then introduces the situation.

You could sometimes change the order, but it would shift the emphasis:

• Столько людей было в магазине, что...
  This emphasizes how many people more strongly.

Russian word order often changes focus rather than basic grammatical meaning.

So the original version sounds smooth and neutral.

Yes, usually that implication is present.

Because of так и не, the sentence suggests:

• trying on the shoes was expected or intended
• but it never happened

So the sentence does not just state a neutral fact. It suggests mild frustration, disappointment, or an unfulfilled intention.

Compare:

• Я не примерила туфли. = I didn’t try on the shoes.
  (plain statement)
• Я так и не примерила туфли. = I never did get to try on the shoes.
  (stronger sense of expectation that wasn’t fulfilled)

That nuance is one of the most important things in this sentence.