Вона купує ще одну марку, бо конверт великий і важкий.

Because марку is in the accusative case. It is the direct object of купує (buys / is buying).

• Dictionary form: марка
• Accusative singular: марку

This happens because feminine nouns ending in -а usually change to -у / -ю in the accusative singular.

So:

• марка = a stamp, a brand
• Вона купує марку = She is buying a stamp

Ще одну means one more or another.

It is made of two parts:

• ще = still, yet, more, another/additional
• одну = one in the feminine accusative singular

Together, ще одну марку literally means one more stamp.

Because марку is feminine accusative singular, одну also has to match it.

Compare:

• одна марка = one stamp
• ще одна марка = one more stamp (as a subject)
• ще одну марку = one more stamp (as an object)

Because одну agrees with марку, which is in the accusative case.

For the numeral/pronoun one, Ukrainian changes the form depending on gender and case.

For feminine singular:

• одна = nominative
• одну = accusative

So:

• Одна марка лежить на столі. = One stamp is lying on the table.
• Вона купує одну марку. = She is buying one stamp.

In your sentence, the stamp is the object, so одну is required.

Бо means because.

Yes, it is very similar to тому що, but бо is usually shorter and a bit more conversational.

So these are both possible:

• Вона купує ще одну марку, бо конверт великий і важкий.
• Вона купує ще одну марку, тому що конверт великий і важкий.

Both mean the same thing: She is buying another stamp because the envelope is big and heavy.

In Ukrainian, the present tense of to be is usually not expressed in sentences like this.

So Ukrainian often says literally:

• конверт великий і важкий
• the envelope big and heavy

But the natural English translation is:

• the envelope is big and heavy

This is completely normal in Ukrainian.

Compare:

• Він лікар. = He is a doctor.
• Кімната тепла. = The room is warm.
• Конверт великий і важкий. = The envelope is big and heavy.

They are in the nominative masculine singular because they describe конверт.

• конверт = envelope
• It is a masculine singular noun
• So the adjectives must match it:
  • великий = big
  • важкий = heavy

This is adjective agreement.

Compare:

• великий конверт = a big envelope
• важкий конверт = a heavy envelope

And after the omitted is, the adjectives still stay nominative:

• Конверт великий і важкий.

Купує is present tense, 3rd person singular, from купувати.

It can mean either:

• she buys
• she is buying

Ukrainian present tense often covers both the simple present and the present continuous, depending on context.

So:

• Вона купує хліб щодня. = She buys bread every day.
• Вона купує хліб зараз. = She is buying bread now.

In your sentence, English would probably most naturally use is buying, but buys is grammatically possible depending on context.

Because конверт is the subject of the second clause:

• бо конверт великий і важкий
• because the envelope is big and heavy

The thing being described is the envelope, so it stays in the nominative.

You can think of the structure as:

• [Subject] + [adjective]
• конверт + великий / важкий

Yes. Марка can have several meanings depending on context, including:

• stamp (especially a postage stamp)
• brand
• make (of a car, for example)

In this sentence, because of конверт (envelope), the meaning is clearly postage stamp.

So here:

• ще одну марку = another stamp

Yes. Ukrainian word order is more flexible than English word order, although different orders can change the emphasis.

The given sentence is neutral and natural:

• Вона купує ще одну марку, бо конверт великий і важкий.

You could also hear variations like:

• Бо конверт великий і важкий, вона купує ще одну марку.
• Ще одну марку вона купує, бо конверт великий і важкий.

These still mean roughly the same thing, but the emphasis shifts:

• putting ще одну марку earlier emphasizes another stamp
• putting the бо clause first emphasizes the reason

Yes. І here simply means and:

• великий і важкий = big and heavy

Ukrainian has both і and й for and. They are often chosen for smooth pronunciation.

For example:

• мама і тато
• він й вона

In your sentence, і is the normal choice.

Usually yes, in this sentence.

But literally, ще одну марку is closer to one more stamp. English often uses another stamp, which sounds more natural.

So these are both good translations:

• She is buying another stamp
• She is buying one more stamp

The Ukrainian phrase is slightly more transparent grammatically:

• ще = more / another
• одну = one
• марку = stamp (accusative)

In Ukrainian, вона means she or it for a feminine noun, depending on context.

Here it is translated as she because the sentence is about a female person buying a stamp.

Ukrainian pronouns work like this:

• він = he / it (masculine)
• вона = she / it (feminine)
• воно = it (neuter)
• вони = they

So grammatically вона could refer to a feminine noun, but in this sentence the intended meaning is clearly she.