Вчера я купил фиолетовый блокнот.

Yes, the word order can change.

Russian word order is more flexible than English because grammatical endings carry a lot of the meaning. Вчера я купил фиолетовый блокнот is a very natural way to say Yesterday I bought a purple notebook.

You could also say:

• Я вчера купил фиолетовый блокнот.
• Фиолетовый блокнот я купил вчера.

These all mean basically the same thing, but the emphasis changes:

• Вчера first puts focus on when
• Я first puts a little more focus on who
• Фиолетовый блокнот first emphasizes what was bought

So the original sentence is normal and common, especially if you are introducing the time first.

Купил is the past tense masculine singular form of the verb купить, which means to buy.

Russian past tense usually agrees with the gender and number of the subject:

• я купил = I bought (said by a male speaker)
• я купила = I bought (said by a female speaker)
• мы купили = we bought
• он купил = he bought
• она купила = she bought
• они купили = they bought

So купил tells you that the speaker is male, or at least that the sentence is being given in the masculine form.

The infinitive is купить.

This is the dictionary form, meaning to buy.

In this sentence:

• купить = to buy
• купил = bought

Also, купить is a perfective verb, so it usually refers to a completed action: the purchase was made.

Its imperfective partner is покупать, which is used for repeated, ongoing, or habitual buying.

For example:

• Вчера я купил блокнот. = Yesterday I bought a notebook. (completed action)
• Раньше я часто покупал блокноты. = I used to buy notebooks often. (habitual action)

Фиолетовый is the adjective purple, and it agrees with the noun блокнот.

Russian adjectives must match the noun in:

• gender
• number
• case

Here, блокнот is:

• masculine
• singular
• accusative case (because it is the direct object)

For an inanimate masculine singular noun, the accusative form looks the same as the nominative. So:

• фиолетовый блокнот = purple notebook

If the noun were feminine, neuter, plural, or animate, the adjective ending could be different.

Examples:

• фиолетовая ручка = purple pen (feminine)
• фиолетовое платье = purple dress (neuter)
• фиолетовые блокноты = purple notebooks (plural)

It is in the accusative, but for this kind of noun, the accusative looks exactly like the nominative.

Блокнот is:

• masculine
• singular
• inanimate

For masculine singular inanimate nouns, the accusative form is the same as the nominative.

So:

• nominative: блокнот
• accusative: блокнот

That is why the word does not visibly change.

Compare:

• Я вижу блокнот. = I see a notebook.
• Я купил блокнот. = I bought a notebook.

But with an animate masculine noun, the accusative usually matches the genitive:

• Я вижу студента. = I see the student.

So nothing unusual is happening here — this is a standard accusative pattern.

Because the noun is inanimate and is the direct object of the verb.

For a masculine singular inanimate direct object, both the noun and adjective usually look like the nominative:

• фиолетовый блокнот

If the noun were animate, then the accusative would usually look like the genitive:

• Я вижу нового студента. = I see the new student.

So:

• фиолетовый блокнот = correct for an inanimate object
• фиолетового блокнота would be the wrong form here

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

So блокнот can mean:

• a notebook
• the notebook

The exact meaning depends on context.

In Вчера я купил фиолетовый блокнот, English would usually say a purple notebook unless the notebook was already known from context.

Russian leaves that distinction unstated unless something else makes it clear.

Often yes, Russian can drop the subject pronoun when it is clear from the verb or context.

So you could say:

• Вчера купил фиолетовый блокнот.

This can mean Yesterday I bought a purple notebook, especially in conversation if the speaker is obvious.

However, я is still very common and natural. Including it can make the sentence clearer or slightly more explicit.

So both are possible:

• Вчера я купил фиолетовый блокнот.
• Вчера купил фиолетовый блокнот.

The version with я is especially helpful for learners because it is clearer.

A very common clue is the ending.

Russian nouns that end in a consonant are often masculine, and блокнот ends in -т.

So:

• блокнот = masculine

That is why the adjective also takes a masculine form:

• фиолетовый блокнот

And that is also why, if the speaker is male, the verb form купил fits nicely with the rest of the sentence.

A helpful stress pattern is:

• вчерá
• я
• купи́л
• фиолéтовый
• блокнóт

So the sentence is pronounced roughly like:

vchy-RA ya koo-PEEL fee-a-LYE-ta-vy blak-NOT

A few notes:

• вчера has stress on the last syllable
• купил has stress on -ил
• фиолетовый has stress on ле
• блокнот has stress on the last syllable

Russian stress is important because it is not always predictable.

Sometimes, but they are not exactly the same.

• блокнот = notebook, notepad, pad for notes
• тетрадь = exercise book, school notebook, copybook

So фиолетовый блокнот suggests a notebook or notepad, while фиолетовая тетрадь would usually mean a different kind of notebook, often one used for schoolwork.

Also notice the grammar change:

• фиолетовый блокнот (masculine)
• фиолетовую тетрадь (feminine accusative singular)

So yes, you can use another noun, but the meaning and grammar may change.

The direct object is фиолетовый блокнот.

It answers the question What did I buy?

• купил что? = bought what?
• фиолетовый блокнот = a purple notebook

In Russian, the direct object often goes in the accusative case, which is exactly what we have here. Because the noun is masculine singular inanimate, the accusative looks the same as the nominative.

So structurally:

• Вчера = adverb of time
• я = subject
• купил = verb
• фиолетовый блокнот = direct object