Вместо риса я сегодня сварю спагетти.

Because вместо requires the genitive case.

• Dictionary form: рис
• Genitive singular: риса

So:

• вместо риса = instead of rice

This is a very common pattern in Russian:

• вместо чая = instead of tea
• вместо кофе = instead of coffee
• вместо урока = instead of a lesson/class

Yes, in normal usage вместо is followed by the genitive case.

Examples:

• вместо хлеба = instead of bread
• вместо меня = instead of me
• вместо книги = instead of the book

So in this sentence, вместо риса is exactly what you should expect grammatically.

Сварю is the 1st person singular future form of сварить.

So it means:

• I will cook
• more literally, I will boil / cook to completion

For food, сварить often means to boil or to cook something until it is ready.

Examples:

• сварить рис = to cook/boil rice
• сварить суп = to cook soup
• сварить яйца = to boil eggs

In this sentence, сварю спагетти means I’ll cook spaghetti.

This is about aspect.

• сварю = perfective future
• буду варить = imperfective future

Сварю focuses on the action as a completed result: the spaghetti will end up cooked.

Буду варить focuses more on the process or ongoing action: I will be cooking / I’ll cook for some time.

Here, сегодня and the context suggest a simple completed plan for today, so сварю sounds very natural.

Compare:

• Я сегодня сварю спагетти. = I’ll cook spaghetti today.
• Я сегодня буду варить спагетти. = I’ll be cooking spaghetti today.
  (more process-oriented, less neutral in this context)

Yes, basically — and that is exactly why it works well here.

In Russian, сварить is a normal verb for cooking foods that are prepared by boiling, including:

• pasta
• rice
• soup
• eggs

So сварить спагетти is a natural Russian way to say cook spaghetti.

English often uses the broader verb cook, while Russian may choose a more specific verb depending on the method.

Because спагетти is an indeclinable noun in Russian.

That means its form usually stays the same in different cases.

Also, in this sentence it is the direct object, so it is in the accusative case. Since it is inanimate, the accusative normally matches the nominative anyway.

So you get:

• спагетти = nominative
• спагетти = accusative

No visible change.

It is usually treated as a plural-form noun in Russian, similar to how the word looks in Italian.

So you will often see plural agreement, for example:

• спагетти были вкусные = the spaghetti were tasty

But in practice, learners mainly need to remember this:

• спагетти does not change form
• it commonly behaves like a plural noun

In your sentence, that does not create any difficulty, because there is no adjective or past-tense verb agreeing with it.

Yes, it could be omitted.

Russian often drops subject pronouns when the verb already makes the subject clear.

• сварю already means I will cook

So both are possible:

• Вместо риса я сегодня сварю спагетти.
• Вместо риса сегодня сварю спагетти.

Including я can:

• add a little emphasis
• make the sentence clearer
• sound slightly more contrastive, depending on context

For example, it can suggest I am the one making that choice.

Yes, Russian word order is flexible.

The sentence:

• Вместо риса я сегодня сварю спагетти.

is natural, but other versions are possible too:

• Сегодня я вместо риса сварю спагетти.
• Я сегодня сварю спагетти вместо риса.
• Спагетти я сегодня сварю вместо риса.

The basic meaning stays the same, but the emphasis shifts.

For example:

• Вместо риса at the beginning highlights the contrast: instead of rice
• Спагетти at the beginning emphasizes what will be cooked
• Сегодня at the beginning emphasizes today

Russian often uses word order to manage focus and emphasis more than English does.

Its basic meaning is simply today.

But in context, especially with cooking, it can naturally imply something like:

• today, for the meal
• today, this time
• today, on today’s menu

Russian often leaves that extra context unstated if it is obvious.

So the sentence does not literally say for lunch or for dinner, but a listener may easily understand that kind of idea from the situation.

Grammatically, yes — вместо can mean instead of / in place of in many contexts.

But in this sentence, because of сварю спагетти, the meaning is clearly about substituting one food for another.

So here it naturally means:

• instead of rice, I’ll cook spaghetti today

not a physical placement meaning.

It is singular genitive.

• dictionary form: рис
• genitive singular: риса

Russian usually treats rice as a mass noun here, just like English does. You are not talking about types of rice or individual grains, just the ingredient in general.

So вместо риса = instead of rice, not instead of rices.

Yes, but the meaning becomes broader.

• спагетти = spaghetti specifically
• макароны = pasta / macaroni / noodles-like pasta in a broader everyday sense

So:

• Вместо риса я сегодня сварю спагетти. = Instead of rice, I’ll cook spaghetti today.
• Вместо риса я сегодня сварю макароны. = Instead of rice, I’ll cook pasta today.

Both are natural; спагетти is just more specific.