Learn Hardcore Icelandic: Hún myndi koma með okkur ef veðrið væri betra. - She would come with us if the weather were better.

Myndi is the past-subjunctive/conditional form of the verb munu and, together with an infinitive (koma), it expresses a hypothetical result in the main clause. It matches the English conditional “would come.” You typically see this when the if-clause uses the past subjunctive (here: ef veðrið væri betra).

Can I say Hún kæmi með okkur instead of Hún myndi koma með okkur?

Yes. Hún kæmi með okkur ef veðrið væri betra is equally correct and perhaps a bit more compact or slightly more formal. It uses the past subjunctive of the main verb (kæmi < koma) instead of the periphrastic conditional myndi + infinitive. Both versions are very common and natural.

Why is it væri and not var after ef?

Because the condition is hypothetical/unreal. Icelandic uses the past subjunctive in such if-clauses: ef [something] væri = “if [something] were.” Using var would refer to a real past situation, not a present counterfactual. Compare:

Hypothetical present: Ef veðrið væri betra, …
Real past tendency: Ef veðrið var betra, fór hún oft með okkur (“If/whenever the weather was better, she often went with us”).

What case is okkur, and why?

Okkur is the dative (and also the accusative) form of við (“we/us”). Here it’s dative because með in the sense of “together with (someone)” governs the dative: með okkur = “with us.”

Does með always take the dative?

No. The meaning decides the case:

Dative for accompaniment: Ég fer með honum (“I go with him”).
Accusative when you have/bring/carry something: Ég kem með bókina (“I’ll bring the book”). In your sentence, it’s accompaniment (“with us”), so dative: með okkur.

Could koma með okkur mean “bring us”?

In practice, hún (myndi) koma með okkur is understood as “she (would) come with us.” To express “bring/take us,” Icelandic usually switches verb or viewpoint:

“take us (along)”: fara með okkur / taka okkur með sér
“bring us (here/to where the speaker is)”: context-dependent, but you’d still prefer verbs like fara með or phrasing with taka … með sér to avoid ambiguity.

Why is it betra and not betri?

Because veðrið (“the weather”) is neuter singular, and comparative adjectives agree with the noun. The neuter nominative/accusative singular of “better” is betra; betri is used for masculine/feminine nominative singular (and elsewhere depending on case/number). So: veðrið væri betra.

Can I put the if-clause first? What happens to commas and word order?

Yes: Ef veðrið væri betra, myndi hún koma með okkur.

Subordinate clause first, then a comma.
The main clause still follows verb-second order: myndi comes before hún.
When the if-clause comes second (as in the original), modern punctuation doesn’t require a comma before ef.

How would I negate it?

Place ekki after the finite verb in the main clause:

Hún myndi ekki koma með okkur ef veðrið væri verra (“She wouldn’t come with us if the weather were worse”). You can likewise negate inside the if-clause: ef veðrið ekki væri svona slæmt (more natural: ef veðrið væri ekki svona slæmt).

How do I say the past counterfactual “would have come … if the weather had been better”?

Use the perfect subjunctive/conditional in both clauses:

Hún hefði komið með okkur ef veðrið hefði verið betra.

Why is it veðrið (definite) and not just veður?

Icelandic commonly uses the definite form when talking about the weather in a specific, current, or general-situation sense: veðrið (“the weather”). You’ll frequently see statements like Veðrið er gott í dag (“The weather is good today”).

Is dropping the subject pronoun possible, like in Spanish/Italian?

No. Icelandic is not a pro-drop language in standard usage. You normally include the subject pronoun: Hún myndi …, not just Myndi …

Can I say ef að instead of just ef?

Colloquially, people sometimes say ef að, but standard and most formal writing prefer simple ef. Your sentence is perfectly natural with ef alone.

Any quick pronunciation tips for tricky letters here?

ð (as in veðrið, með) is like the voiced “th” in English “this.”
Hún roughly “Hoon.”
með roughly “meth” (voiced th).
okkur “OK-kur” (double k is long/geminated).
veðrið roughly “VETH-rith” (roll the r).
væri “VAI-ri.”
betra “BET-ra.”