Я купил две книги в магазине.

Because купил is the past tense masculine singular form of the verb.

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

• купил = a man / masculine subject bought
• купила = a woman / feminine subject bought
• купило = it bought
• купили = they bought

So Я купил usually means I bought, said by a male speaker.

If a female speaker says it, she would say:

• Я купила две книги в магазине.

Yes, often it can, but it depends on context.

Russian often drops subject pronouns when they are already clear. So you may hear:

• Купил две книги в магазине.

However, in the past tense, купил shows gender but not clearly person the way present-tense endings do. So without Я, the meaning can be understood from context, but the sentence is less self-contained than in English.

So:

• Я купил две книги в магазине. = clear, neutral
• Купил две книги в магазине. = natural in conversation if it is already obvious who did it

Because книга is a feminine noun.

The numeral 2 has different forms depending on gender:

• два for masculine and neuter nouns
• две for feminine nouns

Examples:

• два стола = two tables
• два окна = two windows
• две книги = two books

Since книга is feminine, you must say две книги.

Because after the numbers 2, 3, 4, Russian nouns usually go in the genitive singular.

So with книга:

• 1 книга
• 2 книги
• 3 книги
• 4 книги
• 5 книг

That is why:

• две книги = correct

not:

• две книга = wrong
• две книг = wrong here

A useful pattern:

• after 1 → nominative singular
• after 2, 3, 4 → genitive singular
• after 5+ → genitive plural

Because в can take different cases, and here it means in a place, so Russian uses the prepositional case.

• магазин = store
• в магазине = in the store

This sentence answers where?, not to where?

Compare:

• в магазине = in the store, at the store → location
• в магазин = into the store, to the store → direction/movement

So in your sentence:

• Я купил две книги в магазине.
• literally: I bought two books in the store

This is a question about aspect, one of the most important parts of Russian verbs.

• купить / купил = perfective
• покупать / покупал = imperfective

Here купил is used because the sentence presents the action as completed: the books were bought.

• Я купил две книги в магазине. = I bought two books at the store. The result matters: now I have them.

If you said покупал, it would usually suggest something like:

• the action was in progress
• it happened repeatedly
• the focus is on the process, not the result

For example:

• Я покупал книги в этом магазине. = I used to buy books in this store / I was buying books in this store.

So купил is the natural choice for one finished purchase.

No, Russian word order is much more flexible than English word order.

The sentence:

• Я купил две книги в магазине.

is a neutral, natural order.

But Russian can rearrange parts to change emphasis or focus:

• В магазине я купил две книги. = It was in the store that I bought two books.
• Две книги я купил в магазине. = Two books, I bought in the store.
• Я в магазине купил две книги. = also possible, with slightly different rhythm/emphasis

The basic meaning stays similar, but the information structure changes.

For learners, the version you have is the safest neutral one.

No. Russian has no articles.

So книги can mean:

• books
• the books
• some books

and в магазине can mean:

• in a store
• in the store

The exact meaning depends on context.

In English you must choose an article, but in Russian you usually do not mark that distinction directly.

A very common clue is the ending.

Many Russian nouns ending in -а or -я are feminine, and книга ends in -а, so it is feminine.

That matters because it affects other words in the sentence:

• две instead of два
• adjectives would also agree with it

For example:

• интересная книга = an interesting book

There are exceptions in Russian, but книга is a regular feminine noun.

A simple pronunciation guide is:

• Ya kupEEL dvye KNEE-gee v maga-ZEE-nye

More accurately, the stressed syllables are:

• Я купи́л две кни́ги в магази́не.

Stress:

• купи́л
• две
• кни́ги
• магази́не

A few helpful notes:

• Я sounds like ya
• в is a short v
• unstressed vowels are often reduced in normal speech
• г in книги is a hard g

If you want to sound natural, make sure the stress is right: купи́л, кни́ги, магази́не.