I februari är det ofta kallt, så vi stannar gärna inne på kvällen.

In Swedish, months are usually used with i when you mean in a certain month:

• i januari
• i februari
• i mars

So i februari means in February.

Swedish uses på for many time expressions too, but not normally with months.

In Swedish, weather and general conditions often use a dummy subject: det.

So:

• Det är kallt. = It is cold.
• Det regnar. = It is raining.
• Det är mörkt. = It is dark.

The det does not refer to a specific thing. It works like it in English weather expressions.

The adjective kall becomes kallt here because it is used in the expression det är kallt.

This is the neuter/impersonal form, commonly used:

• en kall dag = a cold day
• ett kallt rum = a cold room
• det är kallt = it is cold

So in weather or general-condition phrases with det, Swedish normally uses the -t form.

In a normal main clause, Swedish sentence adverbs and frequency adverbs like ofta usually come after the finite verb.

So:

• Det är ofta kallt.
• Vi stannar ofta inne.

In this sentence, the clause begins with I februari, but the pattern is still basically:

• är = finite verb
• det = subject
• ofta = adverb

Swedish word order is quite structured, and adverbs like ofta commonly appear in that position.

Swedish follows the V2 rule in main clauses: the finite verb must come in the second position.

So when a time expression comes first:

• I februari = first position
• är = second position
• det = comes after the verb

That is why you get:

• I februari är det ofta kallt.

Compare:

• Det är ofta kallt i februari.
• I februari är det ofta kallt.

Both are correct, but when I februari is moved to the front, är must stay in second position.

Så here means so in the sense of therefore / as a result.

The sentence has two linked ideas:

• I februari är det ofta kallt
• så vi stannar gärna inne på kvällen

So the second clause is a consequence of the first: it is often cold, so we prefer to stay inside in the evening.

Here så works like a coordinating conjunction meaning so. After that, Swedish normally starts a new main clause with regular word order:

• så vi stannar gärna inne

That is similar to English:

• ..., so we stay inside ...

If så were placed first as an adverb in a clause on its own, you could get inversion in some contexts, but in this sentence the natural structure is:

• clause 1 + så
  • clause 2

So vi stannar is the expected order here.

Gärna is a very common Swedish word. Here it adds the idea of willingly, gladly, or more naturally in English, like to.

So:

• vi stannar gärna inne
  means something like
• we like to stay inside
• we’re happy to stay inside
• we tend to stay inside willingly

It often sounds more natural in English to translate the whole phrase rather than translate gärna word-for-word.

Compare:

• Jag dricker gärna kaffe. = I like drinking coffee.
• Hon hjälper gärna till. = She is happy to help.

Stanna can mean both stay and stop, depending on context.

Here it means stay:

• vi stannar inne = we stay inside

In other contexts it can mean stop:

• Bussen stannar här. = The bus stops here.

So the meaning comes from the rest of the sentence. Since inne is there, stannar inne clearly means stay inside.

Inne means inside / indoors as a location. It tells you where someone is.

• Vi stannar inne. = We stay inside.

By contrast, in usually expresses movement into something:

• Vi går in. = We go in.

So:

• inne = inside, being there
• in = into, moving there

Since the sentence is about remaining in a place, inne is the correct word.

Yes, you could say inomhus, but it sounds a bit different.

• inne = inside, indoors; very common and natural in everyday speech
• inomhus = indoors; a bit more formal or descriptive

So:

• Vi stannar gärna inne. = very natural everyday Swedish
• Vi stannar gärna inomhus. = also correct, but slightly less conversational

På kvällen means in the evening or in the evenings, depending on context.

Swedish often uses på with parts of the day in expressions like:

• på morgonen = in the morning
• på eftermiddagen = in the afternoon
• på kvällen = in the evening

In this sentence, it suggests a habitual time: in the evening / during the evening.

Good question. They are related, but not the same.

• på kvällen = in the evening / in the evenings, often more general or habitual
• ikväll = tonight / this evening, specific to today

So in this sentence:

• på kvällen fits because the sentence describes a general habit in February.

If you said:

• I februari är det ofta kallt, så vi stannar gärna inne ikväll

that would sound strange, because ikväll refers to one specific evening, while the rest of the sentence is about a general pattern.

Swedish uses the present tense for general truths, habits, and repeated actions, just like English often does.

So here:

• är = is
• stannar = stay

The sentence describes what is often true in February and what we generally do in that situation.

This is why the present tense is the natural choice.