Я купую марку для листівки.

Because марка is the direct object of the verb купую (I am buying / I buy), so it goes into the accusative case.

• Nominative: марка
• Accusative: марку

This happens with many feminine nouns ending in -а:

• книга → книгу
• машина → машину
• листівка → листівку

So Я купую марку means I am buying a stamp.

Because the preposition для (for) requires the genitive case.

• Nominative: листівка
• Genitive: листівки

So:

• для листівки = for a postcard

This is a very common pattern in Ukrainian:

• для друга = for a friend
• для мами = for mom
• для школи = for school

Для takes the genitive case.

In this sentence:

• для
  • листівки
• not для листівка

A good rule to remember is:

• для + genitive = for + noun

Examples:

• для дитини = for a child
• для міста = for the city
• для роботи = for work

It can mean both, depending on context.

Ukrainian present tense often covers both:

• a habitual action: I buy
• an action happening now: I am buying

So Я купую марку для листівки could mean:

• I buy a stamp for a postcard
• I am buying a stamp for a postcard

If you want to make right now clearer, you can add зараз:

• Я зараз купую марку для листівки. = I am buying a stamp for a postcard right now.

Купую is the imperfective form of the verb купувати.

That means it is used for:

• ongoing actions
• repeated actions
• general present-tense statements

Its perfective partner is usually купити.

Compare:

• Я купую марку. = I am buying / I buy a stamp.
• Я куплю марку. = I will buy a stamp.
• Я купив марку. = I bought a stamp.

So in your sentence, купую presents the action as ongoing or general, not as a completed one.

No, Ukrainian word order is fairly flexible.

The neutral order here is:

• Я купую марку для листівки.

But you may also hear:

• Марку для листівки я купую.
• Для листівки я купую марку.

These versions change the focus or emphasis, not the basic meaning.

Still, for learners, the original order is the safest and most natural starting point.

Because Ukrainian has no articles.

So марку can mean:

• a stamp
• the stamp

The exact meaning depends on context.

This is normal in Ukrainian:

• Я бачу будинок. = I see a/the house.
• Вона читає книгу. = She is reading a/the book.

If you need to be more specific, Ukrainian uses context or other words, not articles.

Yes. Марка can also mean brand or make, depending on context.

For example:

• марка автомобіля = car brand / make
• поштова марка = postage stamp

In your sentence, because of для листівки (for a postcard), the meaning is clearly stamp.

If you want to be extra explicit, you could say:

• Я купую поштову марку для листівки.
  = I am buying a postage stamp for a postcard.

A simple pronunciation guide is:

Ya kupUyu mArku dlya lystIvky.

Approximate stress:

• Я
• купУю
• мАрку
• для
• листІвки

A few pronunciation notes:

• я sounds like ya
• ю in купую sounds roughly like yu
• для can feel tricky because of the consonant cluster; say it as one short word, roughly dlya
• и in листівки is not exactly English i; it is a central vowel, somewhat like a relaxed ih

Yes, often you can.

Because the verb form купую already shows I, Ukrainian often drops the subject pronoun when it is clear.

So both are possible:

• Я купую марку для листівки.
• Купую марку для листівки.

The version with Я is still perfectly correct. It may sound slightly more explicit or emphatic, depending on context.

They are different words:

• листівка = postcard / greeting card
• лист = letter

So:

• марка для листівки = a stamp for a postcard
• марка для листа = a stamp for a letter

This is worth noticing because English speakers may expect letter here, but Ukrainian uses a different noun for postcard.

Sometimes на can also be used in stamp-related contexts, but для листівки is a clear and natural way to say for a postcard.

Very roughly:

• для листівки focuses on intended purpose: for a postcard
• на листівку can sometimes sound more like for use on a postcard or depend on the specific context

For a learner, Я купую марку для листівки is a good, safe sentence to use.