К чаю я купил мармелад.

Because к is a preposition, and it requires the dative case here.

• чай = nominative
• чаю = dative

So к чаю literally looks like to tea, but in this kind of sentence it means something like for tea / to go with tea.

This is a very common Russian pattern:

• купить что-то к чаю = to buy something for tea, to have with tea
• что-то к ужину = something for dinner
• подарок к празднику = a present for the holiday

No. Even though к often means toward or up to, in this sentence it does not describe movement.

Here it has an idiomatic meaning: for tea / to go with tea.

So:

• Я подошёл к столу = I walked up to the table → real movement
• Я купил печенье к чаю = I bought cookies for tea → no movement, just purpose or intended use

Russian word order is much more flexible than English word order.

All of these are possible:

• Я купил мармелад к чаю.
• К чаю я купил мармелад.
• Мармелад я купил к чаю.

The basic meaning stays the same, but the emphasis changes.

К чаю я купил мармелад puts к чаю first, so it highlights the occasion or purpose: as for tea, I bought marmalade / a sweet for tea.

A natural English way to feel the emphasis is:

• For tea, I bought marmalade.

Yes, absolutely.

That version is probably the most straightforward neutral order for many learners:

• Я купил мармелад к чаю.

The version with К чаю at the front sounds a bit more organized around the topic tea or what I got for tea.

So the difference is mostly about focus, not core meaning.

In the past tense, Russian verbs show gender in the singular.

• купил = masculine
• купила = feminine
• купило = neuter
• купили = plural

Since the subject is я, the form tells you something about the speaker:

• Я купил = I bought (speaker is male)
• Я купила = I bought (speaker is female)

So this sentence sounds as if the speaker is male.

It is in the accusative case, but for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: мармелад
• accusative: мармелад

That is why there is no visible change.

Compare:

• Я купил мармелад. = I bought marmalade.
• Я вижу стол. = I see a table.

But with an animate masculine noun, accusative often matches genitive:

• Я вижу брата. = I see my brother.

Not exactly. This is a very common false friend.

In Russian, мармелад usually means a jelly-like fruit sweet / gummy fruit candy, not specifically the orange spread called marmalade in English.

So depending on context, a better English idea might be:

• fruit jelly candy
• fruit jellies
• gummy fruit sweets

If a learner translates it automatically as marmalade, that can be misleading.

Russian has no articles.

So мармелад can mean:

• marmalade
• some marmalade
• the marmalade

The exact meaning depends on context.

In this sentence, English might naturally say:

• I bought some marmalade for tea or
• I bought marmalade for tea

Russian simply leaves that unstated.

Yes. Russian often drops personal pronouns when they are clear from the verb or context.

You could say:

• К чаю купил мармелад.

That can sound natural in conversation if it is already obvious who did the buying.

However, because the past tense купил only shows gender, not person, the pronoun я can help make the subject explicit:

• К чаю я купил мармелад.

Including я may also add a little emphasis: I bought marmalade for tea.

Good question. Both can sometimes relate to purpose, but they are not the same.

• к чаю = for tea, to go with tea, as something served with tea
• для чая = for tea, for the sake of tea, intended for tea in a more literal or technical sense

With food and drink, Russian very often uses к to mean something that goes with something else:

• печенье к чаю = cookies for tea
• соус к мясу = sauce for meat
• гарнир к рыбе = a side dish for fish

So к чаю is the natural idiomatic choice here.

Чаю is the regular dative singular form of чай.

The singular forms are:

• nominative: чай
• genitive: чая
• dative: чаю
• accusative: чай
• instrumental: чаем
• prepositional: чае

So after к, you need the dative:

• к чаю

This is a normal masculine noun pattern.

Yes, it can have that broader feeling too.

Depending on context, к чаю may mean:

• something to have with tea
• something for tea time
• a sweet snack for when people sit down to drink tea

Russian often uses this phrase in a very cultural, everyday way: something nice to serve with tea.

So the phrase is not just about the drink itself, but also about the whole tea-snack situation.

Купил is perfective.

The verb pair is:

• покупать = imperfective, to be buying / to buy habitually
• купить → past купил = perfective, to buy, to have bought

Here the speaker is talking about a completed action: the purchase was made.

So:

• Я покупал мармелад к чаю. = I was buying / used to buy marmalade for tea, or I bought it in a process-focused way
• Я купил мармелад к чаю. = I bought marmalade for tea, completed result

In this sentence, the perfective form is the natural choice.