Бабушка купила сладкий рулет к выходным.

Because the subject бабушка is feminine.

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

• купил = he bought
• купила = she bought
• купило = it bought
• купили = they bought

So Бабушка купила means Grandma bought.

Because рулет is a masculine inanimate noun, and here it is the direct object of the verb купила.

For masculine inanimate nouns, the accusative singular looks exactly like the nominative singular:

• nominative: сладкий рулет
• accusative: сладкий рулет

If the noun were animate, the accusative would usually match the genitive instead.

So in this sentence:

• сладкий рулет = the thing Grandma bought

It is in the dative case because the preposition к normally requires the dative.

So:

• к
  • dative

Here:

• выходные becomes выходным

The phrase к выходным often means something like:

• for the weekend
• by the weekend
• in time for the weekend

The exact best translation depends on context.

Because Russian usually expresses weekend as выходные, which is a plural form.

So even when English says the weekend in the singular, Russian often uses a plural noun:

• выходные = weekend / days off

After к, it becomes dative plural:

• к выходным

This is very normal Russian usage.

It means that the sweet roll was bought for the weekend or in preparation for the weekend.

It does not usually mean physical movement toward the weekend in a literal sense. Instead, it suggests purpose or timing:

• Grandma bought it so they would have it on the weekend
• Grandma bought it by the time the weekend comes

So к can sometimes have the idea of by / for a certain time.

Because Russian has no articles.

English distinguishes between:

• a sweet roll
• the sweet roll

Russian usually leaves that unspecified unless the context makes it clear. So сладкий рулет could mean:

• a sweet roll
• the sweet roll

You understand which one is meant from context, not from an article.

Купила is perfective.

Its imperfective partner is покупала.

The difference is:

• купила = bought, completed the action
• покупала = was buying / used to buy / bought in a process-oriented sense

In this sentence, купила is used because the action is presented as a completed event: Grandma went and bought the roll.

Yes. Russian word order is more flexible than English because case endings show grammatical relationships.

The neutral order here is:

• Бабушка купила сладкий рулет к выходным.

But other orders are possible, for example:

• К выходным бабушка купила сладкий рулет.
• Сладкий рулет бабушка купила к выходным.

These versions are grammatically possible, but they shift emphasis:

• starting with к выходным emphasizes the time/purpose
• starting with сладкий рулет emphasizes what was bought

So word order in Russian often affects focus more than basic grammar.

Here рулет means a roll, usually a rolled food item.

Because it is modified by сладкий, it most naturally means a sweet roll or Swiss-roll-type pastry.

Russian рулет can refer to different kinds of rolled foods depending on context, such as:

• a sweet pastry roll
• a meat roll

But сладкий рулет clearly points to a dessert or pastry.

Because adjectives in Russian must agree with the noun they describe in:

• gender
• number
• case

The noun рулет is:

• masculine
• singular
• accusative, but inanimate masculine accusative looks like nominative

So the adjective also appears as:

• сладкий

If the noun changed, the adjective would change too. For example:

• сладкая булочка = feminine
• сладкое пирожное = neuter
• сладкие булочки = plural

Yes, in many contexts на выходные is also possible, but the nuance is a little different.

• к выходным = by/for the weekend, in time for the weekend
• на выходные = for the weekend, for use during the weekend

In everyday speech, both can sometimes work in similar situations. But к выходным especially highlights preparation by a certain time.

So this sentence sounds natural with к выходным because it suggests Grandma bought the roll in advance for the coming weekend.