Barnen lär sig simma i sommar, men jag simmar helst i en lugn sjö.

Barnen means the children. The ending -en makes barn definite plural.

A quick overview:

• ett barn = a child
• barn = children / child in some contexts
• barnen = the children

So Barnen lär sig simma means The children are learning to swim.

Lär sig means learns or is learning, in the sense of acquiring a skill or knowledge for oneself.

The word sig is a reflexive pronoun, similar to oneself. In Swedish:

• lära often means teach
• lära sig means learn

So:

• Jag lär barnen simma = I teach the children to swim
• Barnen lär sig simma = The children learn to swim

That little sig changes the meaning a lot.

Because they have different grammatical roles.

• simma is the infinitive, meaning to swim
• simmar is the present tense, meaning swim / am swimming

In Barnen lär sig simma, the verb simma comes after lär sig and functions like to swim in English.

In jag simmar helst, simmar is the main verb of the clause, so it is in the present tense.

Some Swedish verbs are followed directly by an infinitive without att. Lära sig is one of them.

So Swedish says:

• lära sig simma = learn to swim

not:

• lära sig att simma in this basic pattern

You may sometimes hear att in other contexts or styles, but here the natural form is lära sig simma.

It can express a future-related idea even though the verb is in the present tense.

Swedish often uses the present tense with a time expression to talk about the near future or planned events. So:

• Barnen lär sig simma i sommar can mean The children are learning to swim this summer
• depending on context, it can also feel like The children will learn to swim this summer

The phrase i sommar gives the time frame and helps create that future meaning.

I sommar means this summer or during the summer.

This is a very common Swedish time expression:

• i dag = today
• i kväll = tonight / this evening
• i morgon = tomorrow
• i sommar = this summer

So i here is not literally just in in the English spatial sense. It is part of a natural time expression.

Helst means preferably, rather, or most willingly.

So jag simmar helst i en lugn sjö means something like:

• I prefer to swim in a calm lake
• I would rather swim in a calm lake

It expresses preference.

A useful comparison:

• gärna = gladly / willingly
• helst = preferably / most willingly

Because i means in here, and the speaker is talking about swimming in the water.

• i en sjö = in a lake
• vid en sjö = by a lake
• på en sjö = on a lake, for example in a boat or on the frozen surface

So if you are actually swimming, i is the natural choice.

Because sjö is a common-gender noun, so it takes en.

That gives:

• en sjö = a lake

The adjective must match the noun. For a common-gender singular indefinite noun, the adjective usually has its basic form:

• en lugn sjö = a calm lake

Compare:

• ett lugnt hav = a calm sea
• lugna sjöar = calm lakes
• den lugna sjön = the calm lake

So lugn is correct because sjö is en-gender and singular indefinite.

Because men is a coordinating conjunction, like but in English. It joins two main clauses, and the second clause keeps normal main-clause word order:

• men jag simmar helst...

So the subject jag comes before the verb simmar.

This is different from cases where another element is moved to the front and causes inversion, such as:

• I sommar lär barnen sig simma
• I en lugn sjö simmar jag helst

In those examples, the verb comes before the subject because something else is placed first.

Yes. Barn is one of those Swedish nouns where the indefinite singular and indefinite plural look the same.

So:

• ett barn = a child
• barn = child in some compounds or dictionary form, but also children as an indefinite plural
• barnen = the children

You usually tell singular vs. plural from the article, the verb, or the context.

Usually, yes. In this sentence sjö clearly means lake.

Swedish sjö normally refers to a lake, while hav means sea. So:

• en lugn sjö = a calm lake
• ett lugnt hav = a calm sea

That is why i en lugn sjö sounds like a peaceful inland body of water, not the ocean.