Plásturinn helst vel á sárinu, þó að barnið leiki sér úti.

Because -inn is the suffixed definite article, equivalent to English the.

• plástur = a plaster / bandage
• plásturinn = the plaster / the bandage

In Icelandic, the definite article is usually attached to the end of the noun instead of written as a separate word.

Helst is the 3rd person singular present form of haldast, a middle-voice form related to halda.

Here haldast means something like:

• stay on
• remain attached
• hold up

So Plásturinn helst vel á sárinu means that the plaster stays on the wound well.

This is not the same as the ordinary verb heldur from halda in the sense of holds. The -st form often gives a more intransitive or self-maintaining meaning.

Vel is an adverb meaning well.

It modifies helst, so the sense is that the plaster stays on well or holds well. English often uses well in a similar way with things like:

• It sticks well
• It stays on well

So vel describes how successfully the plaster remains attached.

Because á can take different cases depending on meaning:

• accusative for motion onto something
• dative for location on something

Here the plaster is already located on the wound, so this is a static location, not movement. That is why Icelandic uses the dative:

• sár = wound
• sárinu = the wound, dative singular

So á sárinu means on the wound.

Compare:

• setja plástur á sárið = put a plaster onto the wound
• plásturinn er á sárinu = the plaster is on the wound

Just like plásturinn, sárinu includes the definite article.

The breakdown is:

• sár = wound
• sárið = the wound, nominative/accusative
• sárinu = the wound, dative

So the ending here shows both:

1. definiteness, and
2. case

Icelandic noun endings often carry several pieces of grammar at once.

Þó að means although, even though, or though.

It introduces a subordinate clause that contrasts with the main idea:

• main clause: the plaster stays on well
• subordinate clause: even though the child is playing outside

So the whole sentence expresses that the plaster stays on despite a potentially difficult condition.

Because after þó að, Icelandic often uses the subjunctive.

Here:

• leikur = indicative present, plays
• leiki = subjunctive present

The subjunctive is common after conjunctions like þó að, especially when the clause expresses concession, uncertainty, attitude, or something viewed less as a plain factual statement and more as part of a relationship between clauses.

So þó að barnið leiki sér úti is a very normal structure.

Sér is a reflexive pronoun, here in the dative.

The verb expression is leika sér, which means to play in the sense of amuse oneself / have fun.

So Icelandic often says literally something closer to:

• the child plays itself

But in natural English, you just say the child plays or the child is playing.

This is an important distinction:

• leika sér = play, have fun
• leika
  • object = play something, perform, act out

For example:

• Barnið leikur sér. = The child is playing.
• Hann leikur Hamlet. = He plays Hamlet.

Because úti means outside in the sense of location, while út usually means out/outward in the sense of movement.

Here the child is playing outside, so it is a location:

• úti = outside
• út = out, outward

Compare:

• Barnið leikur sér úti. = The child is playing outside.
• Barnið hleypur út. = The child runs out.

Because this is a subordinate clause introduced by þó að.

In main clauses, Icelandic usually follows the verb-second pattern, where the finite verb tends to come early. But in subordinate clauses, the word order is often more like:

• conjunction
• subject
• verb
• other elements

So:

• Plásturinn helst vel á sárinu = main clause
• þó að barnið leiki sér úti = subordinate clause

That is why barnið comes before leiki.

Yes, barnið literally means the child:

• barn = child
• barnið = the child

In this sentence, the speaker is probably referring to a specific child already known from context, so the definite form is natural.

Icelandic uses the definite form very regularly when the referent is specific. English might sometimes say the child, but depending on context it could also say your child or just imply which child is meant. Icelandic still normally marks definiteness directly on the noun.

Yes. Icelandic often uses the simple present where English might prefer either the simple present or the present progressive.

So barnið leiki sér úti is present tense, and depending on context the English meaning could be:

• the child plays outside
• the child is playing outside

Likewise, helst is present tense and can be translated naturally as:

• stays on
• holds
• keeps staying on

The exact English wording depends on what sounds most natural.