Þegar það er logn og aðeins skýjað, finnst mér best að ganga heim eftir tónleika.

Þegar means when and introduces a subordinate clause: Þegar það er logn og aðeins skýjað.

In Icelandic, the main clause usually follows verb-second word order. That means the finite verb comes in the second slot of the main clause. Since the whole Þegar... clause takes the first slot, the main verb finnst comes next:

• Þegar ... , finnst mér best ...

Not:

• Þegar ... , mér finnst best ...

This is a very common pattern in Icelandic:

• Þegar ég kem, fer ég heim.
• Ef það rignir, verð ég inni.

So the word order here is completely normal.

Here það works like the it in English weather expressions such as it is cold or it is windy. It does not refer to a specific thing; it is just a grammatical subject.

So:

• það er logn = literally it is calm
• það er skýjað = it is cloudy

This is one of the standard ways Icelandic talks about weather.

Logn means calm weather or no wind.

It is a noun, not an adjective in this expression. So Icelandic says:

• það er logn = literally there is calm / it is calm

This is a bit different from English, which often prefers an adjective like calm or still. In Icelandic, weather is often described with set expressions like this.

Skýjað is the neuter singular form, and that is the form often used in impersonal weather expressions.

Compare:

• það er kalt = it is cold
• það er dimmt = it is dark
• það er skýjað = it is cloudy

You can think of this as either:

• agreeing with an unstated neuter idea like veður (weather), or
• simply the normal default form used in weather expressions.

So skýjað is exactly the form you would expect here.

Aðeins often means only, just, or a little. In this sentence it softens the weather description:

• aðeins skýjað = only slightly cloudy, a bit cloudy, or just cloudy

So the idea is not that the sky is heavily overcast, but that it is just somewhat cloudy.

This is a very important Icelandic pattern.

Að finnast often means to seem, to feel, or to think in the sense of personal impression.

So:

• mér finnst = literally to me it seems
• very naturally: I think / I feel

The person who experiences the opinion or feeling is put in the dative case, so Icelandic uses:

• mér finnst = I think / it seems to me
• þér finnst = you think / it seems to you
• honum finnst = he thinks / it seems to him

This is different from the ordinary verb að finna, which means to find or to feel physically in other contexts.

A helpful comparison:

• Ég finn lykilinn. = I find the key.
• Mér finnst þetta gott. = I think this is good / This seems good to me.

The pattern mér finnst best að + infinitive means I think it is best to...

So:

• mér finnst best að ganga heim = I think it is best to walk home

A few useful points:

• best is the superlative form here: best
• Icelandic commonly uses að + infinitive after expressions like this
• the idea is not I walk best, but it seems best to me to walk

Related patterns:

• Mér finnst gott að synda. = I think it is nice to swim.
• Mér finnst betra að bíða. = I think it is better to wait.
• Mér finnst best að fara núna. = I think it is best to go now.

In Icelandic, heim is a very common adverb meaning home / homeward with verbs of motion.

So you say:

• fara heim = go home
• koma heim = come home
• ganga heim = walk home

No preposition is needed.

This is different from heima, which means at home:

• Ég er heima. = I am at home.
• Ég fer heim. = I go home.

So:

• heim = movement toward home
• heima = location at home

There are two things going on here.

1. Tónleikar is a plural-only noun in Icelandic.
Even when English says a concert, Icelandic often uses the plural form tónleikar.

So:

• tónleikar = a concert / concerts, depending on context
• tónleikarnir = the concert / the concerts

This is similar to how English has words like scissors, which are grammatically plural even when referring to one item.

2. After eftir meaning after in time, Icelandic uses the accusative here, so tónleikar becomes tónleika.

So:

• eftir tónleika = after a concert / after concerts
• eftir tónleikana = after the concert / after the concerts

In this sentence, the meaning is probably general or habitual: after concerts, in general, the speaker likes walking home.