Na tej huśtawce dobrze się huśta nawet wtedy, gdy wiatr jest trochę mocniejszy.

Because na here means on, and when na expresses location, Polish normally uses the locative case.

• ta huśtawka = this swing
• na tej huśtawce = on this swing

Both words change because tej has to agree with huśtawce in gender, number, and case.

So:

• nominative: ta huśtawka
• locative: na tej huśtawce

This is a very common pattern:

• na tej ławce = on this bench
• na tej ulicy = on this street

They are closely related words:

• huśtawka = a swing
• huśtać się = to swing, to rock back and forth

So the sentence uses the noun and the related verb:

• na tej huśtawce = on this swing
• się huśta = one swings / it swings

This kind of repetition is completely natural in Polish. It does not sound strange in the way some repetitions might in English.

Because the verb is huśtać się, not just huśtać.

• huśtać can mean to swing something, rock something
• huśtać się means to swing oneself / to be swinging

In this sentence, się is part of the verb meaning. It is not a literal English-style reflexive like oneself every time, but it helps make the verb intransitive and natural in Polish.

So:

• Mama huśta dziecko. = Mom is swinging the child.
• Dziecko się huśta. = The child is swinging.

Yes, in a way. This is a very common general/impersonal Polish construction.

Na tej huśtawce dobrze się huśta does not point to one specific person. It means something like:

• It’s good to swing on this swing
• One swings well on this swing
• This swing is comfortable/good for swinging

Polish often uses 3rd person singular + się to make this kind of general statement.

Compare:

• Tu dobrze się pracuje. = It’s good to work here.
• W tym łóżku dobrze się śpi. = It’s good to sleep in this bed.

So there is no explicit subject like you or people, but the meaning is general.

Because dobrze is an adverb, and it modifies the verb phrase się huśta.

• dobry = good as an adjective
• dobrze = well as an adverb

Here the sentence is describing how it is to swing on that swing, so Polish uses the adverb:

• dobrze się huśta = one swings well / it’s good for swinging

If you used dobry, it would need to describe a noun, not the action.

Nawet wtedy, gdy means even when or more literally even then, when.

It is made of:

• nawet = even
• wtedy = then
• gdy = when

Yes, kiedy is often possible instead of gdy:

• nawet wtedy, kiedy...
• nawet wtedy, gdy...

Both are correct. In many contexts:

• gdy can sound a bit more compact or slightly more formal/written
• kiedy can sound a bit more conversational

But the difference is small here.

This is a very normal use of the comparative in Polish.

Mocniejszy means stronger, and the comparison does not always need to be stated explicitly. The reference point is understood from context: usually stronger than usual, stronger than before, or stronger than ideal.

So wiatr jest trochę mocniejszy means the wind is somewhat stronger than the normal or expected level.

This works like:

• Dziś jest chłodniej. = It’s cooler today.
  (Cooler than yesterday / than usual / than before — context tells you.)
• On jest teraz spokojniejszy. = He’s calmer now.
  (Calmer than earlier.)

Also note:

• trochę mocniejszy = a little stronger
• dużo mocniejszy = much stronger

Yes, that would also be possible, but it would sound a little different.

• wiatr jest trochę mocniejszy focuses on the wind itself as being stronger
• wieje mocniej focuses on the action of blowing blowing more strongly

So you could say something like:

• ...gdy wiatr jest trochę mocniejszy
• ...gdy wieje trochę mocniej

Both are natural. The original sentence is just slightly more descriptive in terms of the wind’s strength as a condition.

Yes. Polish word order is fairly flexible, although not completely free. The original order is natural and neutral:

• Na tej huśtawce dobrze się huśta nawet wtedy, gdy wiatr jest trochę mocniejszy.

You could move parts around for emphasis, for example:

• Nawet wtedy, gdy wiatr jest trochę mocniejszy, na tej huśtawce dobrze się huśta.

This puts more focus on the condition even when the wind is a bit stronger.

In Polish, changing word order often changes emphasis more than basic meaning.

Yes, na is the normal choice here because you sit on a swing, not in it in the usual Polish way of describing it.

So:

• na huśtawce = on a swing

Using w would usually suggest being inside something, which does not fit an ordinary swing very well.

This is similar to:

• na rowerze = on a bicycle
• na krześle = on a chair

So na tej huśtawce is exactly what you would expect.