Learn Hardcore Russian: Я хочу разогреть суп в кастрюле.

греть / нагревать – to heat (general idea, also technical “to heat something up”).
разогревать / разогреть – to heat up something that is already cooked or prepared (reheat food, warm it thoroughly).
подогревать / подогреть – to warm something up a bit, to make it warm/warmer.

For soup or other ready-made food, разогреть is the most standard everyday choice:
разогреть суп, разогреть пиццу, разогреть обед.

You can also say подогреть суп; it often sounds a bit milder (just bring it up to a good eating temperature). греть суп by itself is unusual in everyday cooking speech; it sounds more like describing the general process rather than the goal of having hot, ready-to-eat soup.

Why is the verb разогреть in the infinitive form?

In Russian, хотеть (“to want”) is followed by an infinitive:

Я хочу разогреть суп. – I want to heat up the soup.
Он хочет поесть. – He wants to eat.
Мы хотим поехать в Москву. – We want to go to Moscow.

The infinitive (разогреть) expresses the action that is desired. The conjugated verb is хочу, and разогреть stays in its base (dictionary) form.

Why is разогреть perfective here (разогреть, not разогревать)?

Разогреть is perfective; разогревать is imperfective.

разогреть – a single, complete action: to heat it up (to the end result).
разогревать – an ongoing or repeated process: to be heating up, to heat up regularly, etc.

With хотеть, Russian often uses perfective when you want to express the desire to finish an action successfully:

Я хочу разогреть суп. – I want to heat (and have it hot, ready).
Я хочу прочитать эту книгу. – I want to (finish) reading this book.

Imperfective with хотеть is used when you mean a general, repeated or process-like activity:

Я хочу разогревать суп каждое утро. – I want to heat up soup every morning (habit).

Why is суп in this form? Is it nominative or accusative?

Grammatically it is accusative singular, because it is the direct object of разогреть (What do I want to heat up? – soup).

For inanimate masculine nouns like суп, стол, дом, the accusative singular form is identical to the nominative singular:

Nominative: суп горячий. – The soup is hot.
Accusative: я хочу разогреть суп. – I want to heat up the soup.

So it looks like nominative, but its function here is accusative.

Why is it в кастрюле and not в кастрюлю?

The preposition в can take different cases, with different meanings:

в + accusative → “into” (direction, movement inside).
в + prepositional → “in / inside” (location, where something is).

In the sentence:

в кастрюле – prepositional case → in a pot, inside the pot.

We’re not talking about moving the soup into the pot, we’re talking about heating it while it is in the pot, i.e. location. So we use the prepositional case:

Суп в кастрюле. – The soup is in the pot.
Я хочу разогреть суп в кастрюле. – I want to heat up the soup in the pot.

If you talked about putting soup into the pot, you would use the accusative:

Я наливаю суп в кастрюлю. – I am pouring soup into the pot.

What case is кастрюле and why does it have the ending -е?

Кастрюля is a feminine noun of the -я type:

Nominative singular: кастрюля (a pot, a saucepan).
Prepositional singular: (в) кастрюле.

In the prepositional singular, many feminine nouns ending in -я change to -е:

Россия → в России (in Russia)
Тетрадь → в тетради (in the notebook)
Кастрюля → в кастрюле (in the pot)

So кастрюле is prepositional singular, used after в here to show location (“in the pot”).

Does суп here mean “the soup” or “some soup”? There is no article in Russian.

Russian has no articles, so суп by itself can correspond to:

“the soup”
“some soup”
just “soup” in general

Context decides. In this sentence, in normal conversation, суп would most naturally be understood as “the soup that we know about / that is here”, so English speakers would usually translate it as “the soup”:

Я хочу разогреть суп в кастрюле.
→ “I want to heat up the soup in a pot.”

But grammatically, nothing explicitly marks it as definite or indefinite.

Could I say Я разогрею суп в кастрюле instead of Я хочу разогреть суп в кастрюле? What’s the difference?

Yes, you can, but the meaning changes:

Я хочу разогреть суп в кастрюле.
– I want to heat up the soup in the pot. (expresses desire/intention, not yet a concrete plan or promise)
Я разогрею суп в кастрюле.
– I will heat up the soup in the pot. (a statement about the future; a decision, promise, or prediction)

The second sentence uses the future tense of the perfective verb (разогрею) and presents the action as something that will (almost certainly) happen, not just something you want.

Can I change the word order? For example, can I say Я хочу в кастрюле разогреть суп?

Yes, Russian word order is relatively flexible. All of these are grammatically possible, though the emphasis changes:

Я хочу разогреть суп в кастрюле. – Neutral, most typical.
Я хочу в кастрюле разогреть суп. – Slightly unusual, puts mild focus on “in the pot” (as opposed to, say, in the microwave).
Суп я хочу разогреть в кастрюле. – Emphasis on суп (“As for the soup, I want to heat it up in the pot.”).
В кастрюле я хочу разогреть суп. – Strong focus on “in the pot” (“It’s in the pot that I want to heat the soup.”).

For a beginner or neutral style, Я хочу разогреть суп в кастрюле is the best choice.

Is there a more polite or softer way to say this in Russian?

Yes, you can make it sound less direct or more polite depending on context:

Мне нужно разогреть суп в кастрюле.
– I need to heat up the soup in the pot. (more factual, less about “I want”)
Можно разогреть суп в кастрюле?
– Can (we) heat up the soup in the pot? / Is it okay to heat the soup in the pot?
Я бы хотел(а) разогреть суп в кастрюле.
– I would like to heat up the soup in the pot. (more polite/formal)

The original Я хочу разогреть суп в кастрюле is neutral and completely acceptable in everyday speech.

What is the stress and pronunciation of разогреть and кастрюле?

разогреть: stress is on the last syllable – разогре́ть
Pronounced roughly: [ra-zag-RYET’] (with a soft final ть).
кастрюле: stress is also on the second syllable – кастрю́ле
The ю is

Base form:

кастрюля – кастрю́ля (stress also on -ю-).

Is there any difference in meaning between разогреть суп в кастрюле and разогреть суп на плите?

Yes, slight nuance:

разогреть суп в кастрюле – focuses on the container: the soup is in a pot, and you will heat it in that pot. It doesn’t say how (on the stove, in the oven, etc.), though the stove is implied in real life.
разогреть суп на плите – focuses on the place / appliance: you’ll heat the soup on the stove. It doesn’t say what it’s in (a pot, a pan, etc.), though again a pot is usually implied.

You can also combine them:

Я хочу разогреть суп в кастрюле на плите.
– I want to heat up the soup in the pot on the stove.