Ef veðrið helst gott, gistum við þar aftur í næsta mánuði.

Ef means if and introduces a conditional clause.

So Ef veðrið helst gott is the if-clause, and the rest of the sentence is the main clause.

Helst is the present-tense form of haldast, which means stay, remain, or keep being.

So veðrið helst gott means the weather stays good or remains good.

This is slightly different from a verb meaning become. It suggests the weather is already good and continues that way.

Because veðrið is a neuter singular noun, and the adjective has to agree with it.

• veður = weather
• veðrið = the weather
• gender = neuter

So the adjective must also be neuter singular:

• masculine: góður
• feminine: góð
• neuter: gott

That is why Icelandic says veðrið helst gott.

Because gott is an adjective describing veðrið.

After verbs like vera or haldast in this kind of sentence, Icelandic uses an adjective to describe the subject:

• veðrið er gott
• veðrið helst gott

Vel is an adverb, so it would describe how something is done, not the weather itself.

Veðrið is veður with the definite article attached, so it means the weather.

Icelandic usually attaches the to the end of the noun instead of using a separate word.

• veður = weather
• veðrið = the weather

Here Icelandic, like English, is talking about the weather in a general, known sense.

Because Icelandic very often uses the present tense to talk about the future, especially when the future meaning is already clear from context.

Here the future is clear because of:

• the condition Ef...
• the time phrase í næsta mánuði

So gistum við literally looks present, but in context it means we will stay overnight / we’ll lodge.

Icelandic does have ways to express the future more explicitly, but plain present tense is very common.

This is because Icelandic is a verb-second language.

When the sentence begins with something other than the subject — in this case the whole clause Ef veðrið helst gott — the finite verb of the main clause comes next.

So:

• first position: Ef veðrið helst gott
• second position: gistum
• then the subject: við

That is why you get Ef veðrið helst gott, gistum við...

If the main clause stood alone, you would normally say Við gistum þar aftur...

Gista means stay overnight, spend the night, or lodge somewhere.

It is more specific than just vera (be) or dvelja (stay/remain). It usually implies sleeping there for a night.

So gistum við þar aftur means something like we’ll stay there overnight again.

Because þar means there in the sense of at that place, while þangað means to there / thither, showing motion toward a place.

Here the verb is gista = stay overnight, which describes being at a location, not moving to it.

So:

• þar = there, at that place
• þangað = to that place

That is why þar is the correct choice here.

Here aftur means again.

So þar aftur means there again.

Depending on context, aftur can also mean back, but in this sentence again is the natural meaning.

This means next month.

The preposition í can take different cases depending on meaning. Very roughly:

• accusative often with motion into something
• dative often with location or a time-frame

Here there is no motion. It is a time expression meaning in/during next month, so the dative is used:

• næsti mánuður = next month
• dative singular = næsta mánuði

So í næsta mánuði is the correct form.

Yes, a comma is normally used here.

In Icelandic, when a subordinate clause comes before the main clause, it is typically separated with a comma:

• Ef veðrið helst gott, gistum við þar aftur í næsta mánuði.

This is more regular in Icelandic than in informal English, where commas are sometimes omitted more freely.