Я выбрал синюю ленту, а сестра — красный бантик для подарка.

Выбрал is the past tense masculine singular form of выбрать (to choose).

In Russian past tense, the verb agrees with the subject in gender and number:

• я выбрал = I chose (said by a male speaker)
• я выбрала = I chose (said by a female speaker)
• мы выбрали = we chose

So this sentence suggests that the speaker is male. If the speaker were female, it would be:

Я выбрала синюю ленту, а сестра — красный бантик для подарка.

Синюю is the accusative singular feminine form of синий (blue).

The noun лента is feminine, and here it is the direct object of выбрал (chose what?), so it goes into the accusative case:

• nominative: синяя лента
• accusative: синюю ленту

The adjective must match the noun in:

• gender
• number
• case

So:

• синяя → синюю
• лента → ленту

Because лента is the direct object of the verb выбрал.

Russian changes noun endings depending on their role in the sentence. After a transitive verb like выбрать, the thing chosen usually goes into the accusative case.

For feminine nouns ending in -а, the accusative singular usually changes:

• лента → ленту
• книга → книгу
• машина → машину

So Я выбрал синюю ленту literally means I chose a blue ribbon, with ленту in the correct case.

Because бантик is a masculine inanimate noun, and in the accusative singular, masculine inanimate nouns look the same as the nominative.

So:

• nominative: красный бантик
• accusative: красный бантик

This is different from feminine nouns like лента, which do change visibly in the accusative.

Compare:

• Я вижу красный бантик.
• Я выбрал красный бантик.

Both use accusative, but the form looks like nominative because бантик is masculine and inanimate.

If it were an animate masculine noun, the accusative would match the genitive instead.

The dash marks an omitted verb. In the second part of the sentence, Russian leaves out the verb because it is understood from the first part.

Full version:

Я выбрал синюю ленту, а сестра выбрала красный бантик для подарка.

But Russian often avoids repeating the same verb, especially in comparisons or contrasts. So the shorter version is:

Я выбрал синюю ленту, а сестра — красный бантик для подарка.

The dash helps show that something is missing, roughly like:

I chose a blue ribbon, and my sister — a red bow for the gift.

In natural English, we would restore the verb: ...and my sister chose a red bow for the gift.

А often connects two ideas with a sense of contrast, comparison, or different topic focus, while и is more straightforwardly and.

Here the sentence contrasts two choices:

• I chose a blue ribbon
• my sister chose a red bow

So а is very natural because it means something like:

• whereas
• while
• and as for

Not a strong contradiction, just a contrast.

So:

• Я выбрал синюю ленту, а сестра — красный бантик.
  = I chose a blue ribbon, while my sister chose a red bow.

If you used и, it would sound more like simple addition and less like a comparison between the two people’s choices.

Бантик means a small bow, usually a decorative tied bow made from ribbon.

It is a diminutive form related to бант:

• бант = bow
• бантик = little bow / decorative bow

In this sentence, красный бантик is probably a decorative bow for wrapping a present.

This is not the same as лента:

• лента = ribbon, strip of fabric
• бантик = bow made by tying ribbon

So the sentence contrasts two different gift-decoration items:

• a blue ribbon
• a red bow

Для подарка means for the gift or for a gift.

• для = for
• подарка = genitive singular of подарок (gift/present)

After the preposition для, Russian uses the genitive case:

• подарок → подарка
• друг → друга
• мама → мамы

In this sentence, для подарка most naturally describes бантик:

a red bow for the gift

Depending on context, English might also say for the present or for wrapping the gift.

Because the preposition для requires the genitive.

This is something you mostly have to learn as part of the preposition:

• для кого?
• для чего?

Examples:

• для сестры = for my sister
• для дома = for the house
• для подарка = for the gift

So:

• nominative: подарок
• genitive: подарка

That is why you get для подарка, not для подарок.

Grammatically, it most naturally attaches to красный бантик:

а сестра — красный бантик для подарка
= and my sister [chose] a red bow for the gift

So the most likely meaning is that the sister chose the bow as a decoration for the present.

In real context, a listener might sometimes understand it more broadly, especially if both the ribbon and the bow are for gift wrapping. But by placement, для подарка most directly modifies the second item, бантик.

If you wanted to make it clearly apply to both choices, you might word the sentence differently.

Here are the basic dictionary forms:

• я → я = I
• выбрал → выбрать = to choose
• синюю → синий = blue
• ленту → лента = ribbon
• а → а = and/but/while
• сестра → сестра = sister
• красный → красный = red
• бантик → бантик = little bow
• для → для = for
• подарка → подарок = gift/present

This is useful because Russian dictionaries usually list nouns in the nominative singular and adjectives in the masculine singular nominative.

Yes. Russian word order is flexible, though different orders change the emphasis.

The original sentence is natural:

Я выбрал синюю ленту, а сестра — красный бантик для подарка.

Other possible orders include:

• Синюю ленту выбрал я, а сестра — красный бантик для подарка.
  Emphasis on who chose what.

• Я выбрал для подарка синюю ленту, а сестра — красный бантик.
  This makes for the gift apply more clearly to the first clause.

• Я выбрал ленту синего цвета...
  Possible, but less simple and less natural here.

So the original word order is a good neutral one.

Russian has no articles, so синюю ленту can mean either:

• the blue ribbon
• a blue ribbon

The exact meaning depends on context.

So this sentence by itself does not tell you whether the speaker means a specific ribbon already known to the listener, or simply one blue ribbon among several options.

That is very normal in Russian. Context does the job that a/an/the do in English.