В среду я купил чёрную ручку и серый блокнот для работы.

Because в here means on, and with days of the week Russian uses the accusative case.

• Dictionary form: среда = Wednesday
• Accusative singular: среду

So:

• в среду = on Wednesday

This is just the normal pattern:

• в понедельник = on Monday
• в среду = on Wednesday
• в пятницу = on Friday

Купил is the past tense masculine singular form of купить.

So it tells you two things:

• the action happened in the past
• the speaker is male, or at least the grammatical subject is masculine

Compare:

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

The infinitive is купить = to buy.

This is a question of aspect.

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

Here купил is used because the sentence describes a completed, one-time action with a result: the speaker bought the items.

So:

• я купил ручку = I bought a pen, completed action
• я покупал ручку = I was buying / used to buy / bought in the process sense

In this sentence, the completed purchase is what matters, so купил is the natural choice.

Both nouns are direct objects, so they are in the accusative case. But Russian nouns change differently depending on gender and animacy.

1. ручка is feminine

Its accusative singular changes:

• ручка → ручку

Its adjective must match it:

• чёрная → чёрную

So you get:

• чёрную ручку

2. блокнот is masculine inanimate

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

So:

• серый блокнот stays серый блокнот

That is why one phrase changes more visibly than the other.

Russian adjectives must agree with the nouns they describe in:

• gender
• number
• case

So here:

• чёрную matches ручку

  • feminine
  • singular
  • accusative

• серый matches блокнот

  • masculine
  • singular
  • accusative, which looks like nominative because the noun is inanimate

This agreement is one of the most important patterns in Russian grammar.

It is the genitive singular.

That is because the preposition для always takes the genitive.

• dictionary form: работа
• after для: работы

So:

• для работы = for work

Other examples:

• для дома = for home
• для учёбы = for studying
• для друга = for a friend

It can mean any of those depending on context. Russian often leaves this broader and more natural than English.

So для работы can mean:

• for work
• for my job
• for use at work

Russian does not need to specify my unless that distinction is important.

Because Russian has no articles.

So a noun like ручку can mean:

• a pen
• the pen

and блокнот can mean:

• a notebook
• the notebook

Context tells you which meaning is intended. In this sentence, English would naturally use a black pen and a grey notebook.

No. Russian word order is fairly flexible.

This sentence has a very natural, neutral order:

• В среду = time first
• я = subject
• купил = verb
• then the objects

But other orders are possible:

• Я купил чёрную ручку и серый блокнот в среду
• Чёрную ручку и серый блокнот я купил в среду

These versions are still grammatical, but the emphasis changes.

The original sentence sounds like a neutral On Wednesday, I bought...

Sometimes yes, but not always as naturally.

Russian often drops subjects when they are clear from context. However, in the past tense, the verb shows gender but not person.

So купил tells you the subject is masculine singular, but not specifically I. It could be he too, depending on context.

That is why я is helpful here.

• Я купил = clearly I bought
• Купил by itself = possible in conversation, but more context-dependent

Yes. Ручка commonly means pen.

It can also mean:

• a handle
• a knob
• a small hand in some contexts, though that is less common in everyday standard usage

So Russian learners often notice that ручка has more than one meaning. In this sentence, because it is bought together with a notebook and meant для работы, it clearly means pen.

Most naturally, it refers to the whole idea of the purchase: the speaker bought a black pen and a grey notebook for work.

In real life, both items fit that idea well, so native speakers usually understand it broadly.

Grammatically, a final phrase can sometimes feel closer to the nearest noun, but here the most natural interpretation is that both the pen and the notebook are for work.

If someone wanted to make it absolutely explicit, they might rephrase the sentence.

Yes. In many Russian texts, ё is often written as е, so you may see черную instead of чёрную.

But the pronunciation is still with yo:

• чёрный
• чёрную

For learners, it is very useful to pay attention to ё, because it helps with pronunciation and sometimes avoids confusion.