Jag sitter gärna på balkongen när vädret är varmt.

Swedish uses posture verbs very often in places where English is less specific.

• sitta = sit
• stå = stand
• ligga = lie

So Jag sitter på balkongen can mean literally I am sitting on the balcony, but it can also more loosely mean I sit / spend time on the balcony. In this sentence, it sounds natural and idiomatic in Swedish.

Gärna literally means something like gladly, willingly, or with pleasure. In many sentences, a very natural English translation is like to.

So:

• Jag sitter gärna på balkongen
  = I like sitting on the balcony
  or I happily sit on the balcony

It does not mean please here.

In a normal Swedish main clause, the finite verb usually comes early, and gärna commonly comes after it.

So:

• Jag sitter gärna ...

This is the natural word order. If you put gärna somewhere else, it can sound unusual or change the emphasis.

Because in Swedish, you are typically on a balcony, not in it.

• på balkongen = on the balcony

Swedish often uses på for surfaces, platforms, and places you are positioned on. A balcony is thought of as a space you stand or sit on.

Because balkongen is the definite form: the balcony.

In Swedish, the definite article is often added to the end of the noun:

• en balkong = a balcony
• balkongen = the balcony

So på balkongen means on the balcony.

For the same reason: vädret is the definite form, meaning the weather.

• ett väder = a weather
• vädret = the weather

In practice, weather is very often definite in Swedish when talking about the weather in general, just as in English.

Here när means when or whenever.

• när vädret är varmt = when the weather is warm

This gives a general condition or recurring situation. It suggests that whenever the weather is warm, the speaker likes to sit on the balcony.

If you used om, it would mean if, which sounds more conditional:

• om vädret är varmt = if the weather is warm

Because vädret is a neuter noun, an ett word.

When an adjective is used after är with a neuter singular noun, it usually takes -t:

• vädret är varmt = the weather is warm

Compare:

• dagen är varm = the day is warm
• vädret är varmt = the weather is warm

So the adjective agrees with the noun.

Because när vädret är varmt is a subordinate clause, and Swedish word order changes in subordinate clauses.

In a main clause, Swedish usually follows the verb-second pattern:

• Vädret är varmt

But after a subordinating word like när, the subject normally comes before the verb:

• när vädret är varmt

So this word order is exactly what you should expect.

Usually it sounds like a general habit or preference, not necessarily something happening at this exact moment.

• Jag sitter gärna på balkongen när vädret är varmt
  suggests I like to sit on the balcony when the weather is warm

So the present tense here often expresses something habitual or generally true.

Yes, absolutely. That is also correct and natural.

There is a small difference in feel:

• Jag sitter gärna ... = a lighter, very idiomatic way to say you like doing it
• Jag gillar att sitta ... = a more direct I like to sit ...

Both work, but gärna is very common in Swedish and worth getting used to.

Not usually. A word-for-word translation is possible, but it often sounds unnatural in English.

In many cases, a better English translation is:

• like to sit
• enjoy sitting
• am happy to sit

So when you see gärna with a verb, it is often better to think about the natural English meaning rather than translating each word separately.