Learn Hardcore Swedish: Det är trappan som barnen leker i när det regnar ute. - It is the stairs that the children play in when it is raining outside.

Det är trappan som barnen leker i ... ≈ It is the stairs that the children play in ...
The cleft structure is used to emphasize or contrast the noun trappan: that place, and not some other place.
The more neutral version is simply: Barnen leker i trappan när det regnar ute.
So the meaning is essentially the same, but the cleft form highlights trappan as the important or contrastive information.

Why is it Det är trappan ... and not Den är trappan ..., since trappan is common gender?

Here det is a dummy subject, not a pronoun standing for trappan.

In cleft sentences, Swedish almost always uses det:
- Det är jag som ringer. – It’s me who is calling.
- Det är boken som jag vill ha. – It’s the book that I want.
The det in this pattern does not agree in gender with the focused word (trappan). It’s a fixed construction: Det är X som ....

So you should not say Den är trappan som ...; that would be ungrammatical.

What exactly is som doing in som barnen leker i?

Som is a relative pronoun here, similar to English that/which.

It refers back to trappan.
Inside the relative clause som barnen leker i, the structure is:
- subject: barnen
- verb: leker
- preposition: i
- and som is the thing they play in (the complement of i).

So you can think of it as:

Det är trappan som barnen leker i.
≈ It’s the stairs that the children play in.
where som ≈ that.

Is it really okay in Swedish to put the preposition i at the end, like som barnen leker i?

Yes, this is normal and very common in Swedish, especially in relative clauses.

Det är trappan som barnen leker i. – completely natural.
A more formal alternative would move the preposition in front of a more explicit relative word:
- Det är trappan i vilken barnen leker. (formal/old-fashioned)

You cannot say Det är trappan i som barnen leker; som cannot follow the preposition in that way.
So for everyday Swedish, ending the clause with the preposition (... som barnen leker i) is the standard choice.

Could you say Det är trappan där barnen leker ... instead of som barnen leker i?

You can, but it changes the structure slightly.

som is a relative pronoun: trappan som barnen leker i = the stairs that the children play in.
där is a relative adverb meaning where: trappan där barnen leker = the stairs where the children play.

Both are grammatically possible, but:

With a cleft, Det är trappan som barnen leker i ... sounds more neutral and idiomatic.
Det är trappan där barnen leker ... is understandable and acceptable, but many speakers would still prefer som here.

So som is the safest, most typical choice in this kind of cleft sentence.

Why is it leker i trappan and not leker på trappan?

Both i trappan and på trappan are possible, but they point to slightly different images.

leka i trappan – literally play in the stairs, usually in the sense of inside a stairwell or in a staircase area of a building.
leka på trappan – play on the steps, often the steps outside a house (the front steps / porch).

In many contexts speakers do say leka i trappan even if the children are just on the steps, but as a rule of thumb:

i trappan = within a stairwell / staircase area
på trappan = on the surface of the steps, especially outside

The sentence you have suggests a specific staircase area the children use when it rains.

Why does the sentence say det regnar ute? Isn’t det regnar enough?

Det regnar by itself is completely correct Swedish.

Adding ute makes it more explicit that it is raining outside, and it is very idiomatic in everyday speech:

Det regnar. – It’s raining.
Det regnar ute. – It’s raining outside.

In many situations the ute doesn’t add much new information (where else would it rain?), but it sounds natural and can give a slightly more vivid, concrete feel to the sentence.

What is the role of det in det regnar? Is it actually referring to something?

No, here det is also a dummy subject, like English it in it rains.

Swedish requires a subject in a finite clause, so for weather expressions it uses det:
- Det regnar. – It’s raining.
- Det snöar. – It’s snowing.
- Det blåser. – It’s windy.

This det does not refer to any concrete thing; it just fills the subject position for grammatical reasons.

Why is the word order när det regnar ute and not när regnar det ute?

Because när det regnar ute is a subordinate clause, not a question.

In Swedish main clauses normally have verb in second position (V2):
- Idag regnar det.
But in a subordinate clause introduced by a conjunction like när, the order is typically:
- [conjunction] + subject + verb + ...

So:

när det regnar ute – correct as a subordinate clause (when it rains outside).
När regnar det ute? – is a question (When does it rain outside?).

In your sentence, när det regnar ute is not a question, so the subject det comes before regnar.

Why are trappan and barnen in the definite form?

Swedish marks definiteness with suffixes:

trappa (a stair / a staircase) → trappan (the stair / the staircase)
barn (child / children) → barnen (the children)

Here:

trappan indicates a specific, identifiable staircase (both speaker and listener know which one).
barnen indicates specific children, not children in general.

This matches the English meaning with the: the stairs and the children.

Could I say Det är i trappan som barnen leker när det regnar ute instead? Does that change the meaning?

Yes, you can, and the meaning is very close, but the focus shifts slightly.

Det är trappan som barnen leker i ...
- Focuses more on trappan as the important element.
Det är i trappan som barnen leker ...
- Focuses more on the place phrase i trappan (the location), as opposed to some other location.

Both are natural; both essentially say that the relevant place is the staircase. The second version feels a bit more like It’s in the stairs that the children play ... with strong emphasis on the in the stairs part.

Why is the verb leka used and not spela?

Swedish distinguishes these two verbs:

leka – to play in the sense of children’s free play, imaginative play, animals playing, etc.
- Barnen leker i trappan. – The children are playing in the stairs.
spela – to play structured games, sports, or musical instruments.
- spela fotboll, spela kort, spela piano.

Since the sentence is about children playing in a place (not playing a game like football or cards), leka is the correct verb.