På gården leker barnen gärna i sandlådan eller på gungan när gräset är torrt.

This is because of the Swedish V2 rule: in a main clause, the finite verb usually comes in the second position.

So the basic order would be:

• Barnen leker gärna ... = The children gladly/like to play ...

But when you move another element to the front, such as På gården:

• På gården leker barnen gärna ...

Now På gården takes the first position, so the verb leker must come next, before the subject barnen.

This is very common in Swedish.

Gärna is a very common Swedish adverb. It often means:

• gladly
• with pleasure
• like to
• sometimes preferably, depending on context

In this sentence, barnen leker gärna means something like:

• the children like to play
• the children happily play
• the children often enjoy playing

So gärna adds the idea that they do it willingly or with enjoyment.

In Swedish, på gården is the normal expression for being in/on the yard, courtyard, schoolyard, playground area, depending on context.

Even though English usually says in the yard, Swedish idiomatically uses på here.

So:

• på gården = in the yard / in the courtyard / on the playground

This is one of those cases where Swedish prepositions do not match English exactly and just have to be learned as fixed usage.

The prepositions match the physical relationship:

• i sandlådan = in the sandbox
• på gungan = on the swing

You are thought of as being inside the sandbox area, so Swedish uses i.

You are on the swing, so Swedish uses på.

This is quite similar to English in this case.

These are the definite forms in Swedish. Instead of usually putting a separate word like the before the noun, Swedish often adds the definite article as an ending.

In this sentence:

• barnen = the children
• sandlådan = the sandbox
• gungan = the swing
• gräset = the grass

A quick breakdown:

• barn = children / child(ren) in base form depending on number

• barnen = the children

• sandlåda = sandbox

• sandlådan = the sandbox

• gunga = swing

• gungan = the swing

• gräs = grass
• gräset = the grass

So those endings are doing the job that the does in English.

Barn is a special noun in Swedish.

• ett barn = a child
• barn = children
• barnet = the child
• barnen = the children

So the indefinite plural is the same as the singular base form:

• singular: barn
• plural: barn

Only the article or context makes the number clear.

That is why barnen is clearly the children.

Because gräs is an ett-word, and Swedish adjectives must agree with the noun.

The adjective torr changes like this:

• en word: torr
• ett word: torrt
• plural: torra

Since gräset is the definite form of ett gräs, the adjective becomes torrt:

• gräset är torrt = the grass is dry

This agreement also happens after är with predicate adjectives, which is something English does not do.

Gård can mean different things depending on context, for example:

• yard
• courtyard
• schoolyard
• farmyard

In this sentence, because there is a sandbox and a swing, gården probably refers to some outdoor area where children play, such as:

• a yard
• a playground area
• a schoolyard
• the courtyard of a building

So the exact English word depends on the context.

It is a time clause introduced by när, which means when.

So:

• när gräset är torrt = when the grass is dry

This clause tells us under what condition or at what time the children like to play there.

Also notice the word order inside this subordinate clause:

• gräset är torrt

That is normal subordinate-clause order in Swedish.

Yes.

• leka = to play
• leker = play / are playing

In Swedish, the present tense often covers both meanings that English separates:

• The children play
• The children are playing

So barnen leker can mean either, depending on context. Here it has a general/habitual meaning because of gärna and the rest of the sentence.

Yes. You could say:

• När gräset är torrt leker barnen gärna i sandlådan eller på gungan.

That means the same thing.

Again, the V2 rule applies in the main clause: after the fronted subordinate clause När gräset är torrt, the verb leker comes before the subject barnen.

So this is correct:

• När gräset är torrt leker barnen gärna ...

Not:

• När gräset är torrt barnen leker gärna ...

Because Swedish usually expresses definiteness by adding it to the noun itself.

So instead of:

• the children
• the sandbox
• the swing
• the grass

Swedish says:

• barnen
• sandlådan
• gungan
• gräset

A separate definite word can appear in Swedish too, but usually when there is an adjective:

• den stora gungan = the big swing
• det torra gräset = the dry grass
• de glada barnen = the happy children

In your sentence, there are no adjectives before those nouns, so the ending alone is enough.