Рис сварится через двадцать минут.

Because свариться is a perfective verb. In Russian, perfective verbs do not have a true present tense. Their “present-looking” forms express the future.

So:

• сварится = will get cooked / will be ready
• not is getting cooked

This is a very common Russian pattern:

• сделает = will do
• напишет = will write
• сварится = will cook / will be cooked

Because сварится focuses on the result: the rice will finish cooking.

Compare:

• Рис сварится через двадцать минут.
  = The rice will be cooked / ready in twenty minutes.

• Рис будет вариться через двадцать минут.
  = The rice will be cooking in twenty minutes.

So сварится is about completion, while будет вариться is about the process.

Not literally. Here -ся makes the verb work more like an intransitive or middle-voice verb.

Compare:

• Я сварю рис. = I will cook the rice.
  Here сварю is transitive: someone cooks something.

• Рис сварится. = The rice will get cooked / will be ready.
  Here the rice is the subject, and the sentence does not mention the cook.

So -ся here is not really “the rice cooks itself” in a literal sense. It is just a normal Russian way to say that something comes into a cooked state.

Because in this sentence рис is the subject.

The verb сварится is intransitive here, so there is no direct object. The thing undergoing the change is the grammatical subject:

• Рис сварится. = The rice will be cooked.

Compare with a transitive version:

• Я сварю рис. = I will cook the rice.

In that sentence, рис is the object. But in Рис сварится, it is the subject, so nominative is correct.

It means twenty minutes from now or after twenty minutes.

So the idea is:

• now → 20 minutes pass → the rice is ready

This is the normal meaning of через + time expression:

• через час = in an hour
• через неделю = in a week
• через пять минут = in five minutes

It does not primarily describe the length of the cooking process; it gives the time after which the result will happen.

Because через takes the accusative case, and with numerals Russian uses special patterns.

Here is what is happening:

• через requires accusative
• двадцать is in the accusative form (which looks the same as the nominative here)
• after numerals like двадцать, the noun is usually genitive plural
• so we get двадцать минут

That is why the phrase is:

• через двадцать минут

Similar examples:

• через десять дней
• через тридцать секунд
• через пять лет

Sometimes, but the meaning changes.

• через двадцать минут = twenty minutes from now
• за двадцать минут = in the space of twenty minutes / taking twenty minutes

So:

• Рис сварится через двадцать минут.
  = The rice will be ready twenty minutes from now.

• Рис сварится за двадцать минут.
  = The rice will cook in twenty minutes (that is how long it takes).

In English, both can sometimes be translated as in twenty minutes, which is why learners often confuse them. Russian keeps the two meanings more separate.

Yes, that is also correct.

Russian word order is fairly flexible, and changing it often changes focus rather than core meaning.

Compare:

• Рис сварится через двадцать минут.
  Focuses a bit more on the rice

• Через двадцать минут рис сварится.
  Focuses a bit more on the time

Both are natural. The difference is mostly about emphasis and information structure, not grammar.

A helpful stress guide is:

• Рис сварИтся чЕрез двАдцать минУт.

A few pronunciation notes:

• сварИтся has stress on И
• чЕрез has stress on the first syllable
• двАдцать has stress on the first syllable
• минУт has stress on the last syllable

Also, in normal speech, -тся is often pronounced roughly like -ца:

• сварИтся sounds roughly like сварИца

That is pronunciation only; the spelling remains -тся.

Yes, it is natural.

It is a normal way to say that rice will be cooked after a certain amount of time. Native speakers might also say:

• Рис будет готов через двадцать минут.
  = The rice will be ready in twenty minutes.

That version is a bit more neutral and focuses on readiness rather than the cooking action itself. But Рис сварится через двадцать минут is perfectly normal if you want to talk specifically about the rice finishing cooking.