На обед мама приготовила индейку, а на гарнир — рис с овощами.

Because the subject is мама, which is grammatically feminine. In the past tense, Russian verbs agree with gender and number:

• он приготовил = he prepared
• она приготовила = she prepared
• они приготовили = they prepared

So мама приготовила means Mom prepared.

Because it is the direct object of the verb приготовила. Russian uses the accusative case for direct objects.

• nominative: индейка = turkey
• accusative: индейку = turkey as the thing being prepared

This is a very common pattern for feminine nouns ending in -а:

• мама → маму
• книга → книгу
• индейка → индейку

It is also a direct object, but рис is an inanimate masculine noun. For many inanimate masculine nouns, the accusative is identical to the nominative.

So:

• nominative: рис
• accusative: рис

That is why you see приготовила рис, not a visibly different form.

На обед means for lunch or for the midday meal.

In Russian, на + meal/event is often used to mean for that occasion:

• на завтрак = for breakfast
• на обед = for lunch
• на ужин = for dinner

So На обед мама приготовила... means For lunch, Mom prepared...

На гарнир means as a side dish or for the side.

Гарнир is a side dish that accompanies the main dish. So in this sentence:

• индейку = the main dish
• рис с овощами = the side dish

This expression is very common in food-related Russian.

The dash shows that something has been omitted, usually because it is understood from the first part of the sentence.

Full version:

• На обед мама приготовила индейку, а на гарнир приготовила рис с овощами.

Russian often leaves out repeated words when they are obvious. The dash marks this omission neatly:

• ..., а на гарнир — рис с овощами.

So the second part means:

• and as a side dish, (she prepared) rice with vegetables

А often links two parts while also contrasting or setting them side by side. It is very natural here because the sentence is dividing the meal into two parts:

• main dish: индейку
• side dish: рис с овощами

So а here is like:

• and as for the side...
• while for the side...

Using и would sound more like simple addition. А gives a clearer contrast between the main item and the side item.

Because the preposition с meaning with requires the instrumental case.

• овощи = vegetables (nominative plural)
• овощами = with vegetables (instrumental plural)

So:

• рис с овощами = rice with vegetables

This is a very important pattern:

• чай с сахаром = tea with sugar
• бутерброд с сыром = sandwich with cheese
• суп с овощами = soup with vegetables

Its normal meaning is rice with vegetables, where the vegetables are part of the rice dish or served together with it.

If you simply wanted to say rice and vegetables, Russian would more directly use рис и овощи.

So с usually suggests accompaniment or combination, not just a plain list.

Yes, but it is also very natural Russian word order.

• На обед comes first to set the context: for lunch
• мама is the subject
• приготовила is the verb
• индейку is the object

So the sentence starts with the occasion, then tells who did it and what was prepared.

Russian word order is flexible, but changes emphasis. This version sounds smooth and neutral:

• На обед мама приготовила индейку...

Other orders are possible, but they may highlight different parts of the sentence.

Yes. A fuller version would be:

• На обед мама приготовила индейку, а на гарнир приготовила рис с овощами.

This is grammatically correct, but a bit repetitive. Russian prefers to omit the repeated verb when it is obvious.

That is why the original version with the dash sounds more natural.

Приготовила is perfective. That means the action is presented as completed: she prepared it, and the food is ready.

Compare:

• готовила = was preparing / used to prepare / prepared in an ongoing sense
• приготовила = prepared, finished preparing

In this sentence, perfective makes sense because it is about the result: the lunch menu was made.

Not with exactly the same meaning.

• на обед = for lunch, as the lunch meal
• к обеду = by lunchtime / for lunchtime / in time for lunch

So:

• На обед мама приготовила индейку = Mom prepared turkey for lunch.
• К обеду мама приготовила индейку = By lunchtime, Mom had prepared turkey.

They are close, but к обеду focuses more on timing, while на обед focuses more on purpose.