I värmen dricker jag hellre vatten än kaffe.

I värmen means in the heat or when it’s hot. It sets the scene first, like saying:

• In hot weather, I’d rather drink water than coffee.

Swedish often puts a time or situation phrase at the beginning of the sentence for emphasis or flow.

I värmen is an idiomatic expression. Here, värmen is the noun heat, so the phrase literally means in the heat.

• varm is an adjective meaning warm, so i varm would not work.
• när det är varmt also works, and means when it is warm, but it is a fuller clause.

So:

• I värmen dricker jag hellre vatten än kaffe. = more compact, idiomatic
• När det är varmt dricker jag hellre vatten än kaffe. = also correct, slightly more explicit

Swedish often uses the definite form in expressions like i värmen, i solen, i regnet, where English might just use a general phrase like in the heat, in the sun, in the rain.

Here:

• värme = heat
• värmen = the heat

Even though English and Swedish do not always use definiteness in exactly the same way, i värmen is the natural Swedish expression.

This is because of the Swedish V2 rule: in a main clause, the finite verb usually comes in the second position.

The sentence begins with I värmen, which takes the first position. That means the verb dricker must come next, and the subject jag comes after it.

So:

• Jag dricker hellre vatten än kaffe.
• I värmen dricker jag hellre vatten än kaffe.

Both are correct, but when something other than the subject comes first, Swedish usually switches to this verb-before-subject order.

Hellre means rather, preferably, or would rather.

In this sentence, it shows preference:

• dricker jag hellre vatten än kaffe = I would rather drink water than coffee

It is the comparative form of gärna:

• gärna = gladly / willingly
• hellre = rather
• helst = preferably / most gladly

For example:

• Jag dricker gärna kaffe. = I like drinking coffee.
• Jag dricker hellre vatten. = I’d rather drink water.
• Jag dricker helst vatten. = I prefer water most of all.

Swedish can use a verb like föredra, but hellre is very common and natural in everyday speech.

Compare:

• I värmen dricker jag hellre vatten än kaffe.
• I värmen föredrar jag vatten framför kaffe.

Both are correct, but the version with hellre often sounds more conversational.

Än means than in comparisons.

Since hellre expresses preference, Swedish uses än to compare the two options:

• hellre vatten än kaffe = rather water than coffee

This is parallel to English:

• I’d rather drink water than coffee.

Because Swedish, like English, often leaves out repeated words when they are understood from context.

So:

• hellre vatten än kaffe

really means something like:

• hellre vatten än kaffe
• understood: hellre vatten än kaffe att dricka
• or more fully: hellre dricka vatten än dricka kaffe

But repeating the verb would sound unnecessary in this sentence.

Because vatten and kaffe are being used as uncountable substances here, just like water and coffee in English.

So Swedish naturally says:

• dricka vatten
• dricka kaffe

not usually:

• dricka ett vatten
• dricka ett kaffe

unless you mean a specific serving in a café-style context. For example:

• Jag tar ett kaffe. = I’ll have a coffee.

But in your sentence, the meaning is general, so no article is used.

Yes. Using the definite forms kaffet or vattnet would make it sound more specific:

• kaffe = coffee in general
• kaffet = the coffee
• vatten = water in general
• vattnet = the water

In this sentence, the speaker is talking about general preference in hot weather, so the indefinite mass-noun form is the natural choice.

Yes, that is possible, but the emphasis changes a bit.

• I värmen dricker jag hellre vatten än kaffe.
  This highlights the condition first: in the heat
• Jag dricker hellre vatten än kaffe i värmen.
  This starts with I, so it feels a bit more neutral or subject-focused

Both can be grammatical, but the first version is especially natural if you want to emphasize the situation.

It usually goes after the finite verb in a main clause.

Here:

• dricker = finite verb
• hellre comes after it

So:

• Jag dricker hellre vatten än kaffe.
• I värmen dricker jag hellre vatten än kaffe.

That placement is very typical.

Not exactly.

• i värmen = in the heat / when it’s hot
• på sommaren = in the summer

Summer may be hot, but not always, and heat can happen outside summer too. So i värmen focuses on temperature, not season.

It sounds completely natural and normal in everyday Swedish. It is neither especially formal nor slangy.

A native speaker could easily say this in conversation, and it would also look fine in writing.