Det regnar, så vi stannar inne i kväll.

In Swedish, weather expressions often use det as a dummy subject, just like English uses it in It’s raining.

So:

• Det regnar = It is raining
• Det snöar = It is snowing
• Det blåser = It is windy / It is blowing

The det does not refer to a specific thing. It is just required by the grammar.

Here, så means so or therefore.

It links the two clauses:

• Det regnar = one situation
• så vi stannar inne i kväll = the result of that situation

So the structure is:

• It’s raining, so we’re staying inside tonight

In everyday Swedish, så is very common for this meaning.

Because så here is being used as a coordinating conjunction, like English so. After a coordinating conjunction, Swedish usually keeps normal main clause word order:

• vi stannar = subject + verb

So:

• Det regnar, så vi stannar inne i kväll.

If you use a different connector, such as därför, the word order changes:

• Det regnar. Därför stannar vi inne i kväll.

So this is an important contrast:

• så vi stannar
• därför stannar vi

Stanna means stay or remain.

In this sentence, vi stannar inne means we stay inside or we remain indoors.

A few examples:

• Vi stannar hemma. = We’re staying home.
• Kan du stanna här? = Can you stay here?
• Bussen stannar här. = The bus stops here.

So stanna can mean either stay or stop, depending on context.

Inne means inside / indoors and is the natural word here.

In Swedish, in and inne are different:

• in usually shows movement into something
  
  • Vi går in. = We go in.
• inne describes being inside
  
  • Vi är inne. = We are inside.

Since the sentence is about staying inside, not going inside, Swedish uses inne:

• vi stannar inne

Both i kväll and ikväll are correct and common, and both mean tonight / this evening.

You will see both spellings:

• i kväll
• ikväll

In modern Swedish, the one-word form ikväll is very common, especially in everyday writing. The two-word form i kväll is also accepted and often appears in more careful or traditional writing.

So in this sentence, either of these works:

• Det regnar, så vi stannar inne i kväll.
• Det regnar, så vi stannar inne ikväll.

Swedish often uses the present tense to talk about the near future, especially when the time is clear from context.

Here:

• regnar = present
• stannar = present
• i kväll tells you when

So the sentence can naturally mean something like:

• It’s raining, so we’re staying inside tonight

This is very similar to English, which also often uses the present continuous or present for planned/expected situations.

Compare:

• Vi åker i morgon. = We’re leaving tomorrow.
• Hon kommer i kväll. = She’s coming tonight.

Yes, you could, but the meaning changes slightly.

• vi stannar inne = we stay inside / indoors
• vi stannar hemma = we stay home

Inne focuses on being indoors, while hemma focuses on being at home.

So:

• If you mean you will not go outdoors, inne fits well.
• If you mean you will not leave home, hemma may fit better.

Sometimes both make sense, but they are not exactly identical.

The comma separates two main clauses:

• Det regnar
• så vi stannar inne i kväll

In Swedish, a comma before så is often used when it connects two full clauses, especially in clear written style. In less formal writing, punctuation can vary, but this comma is completely natural and standard.

No. They refer to different parts of the day.

• i kväll = this evening / tonight
• i natt = tonight in the sense of during the night

So:

• Vi stannar inne i kväll = we stay inside this evening / tonight
• Det regnade i natt = it rained last night / during the night

English tonight can sometimes match either one, so Swedish is often more specific.

The sentence as given is already natural. In everyday speech, you might also hear:

• Det regnar, så vi stannar inne ikväll.
• Det regnar, så vi är inne ikväll.
• Det regnar, så vi stannar hemma ikväll.

The exact choice depends on whether the speaker wants to emphasize:

• staying inside = stannar inne
• being inside = är inne
• staying at home = stannar hemma

But the original sentence is perfectly idiomatic.