Learn Hardcore Icelandic: Við vonum að veðrið verði gott.

Because this is a subordinate clause after a verb of hoping. Icelandic normally uses the subjunctive mood (viðtengingarháttur) in such clauses. Verði is the present subjunctive of verða (“to become/be”), and it conveys a desired or uncertain outcome. Using the indicative verður would sound more like a factual assertion (“that the weather will be good”), which clashes with the idea of hope. The most idiomatic choice after vona is the subjunctive: að veðrið verði gott.

Is að here the same að as the English “to,” or does it mean “that”? Does it force the subjunctive?

Here að is a complementizer meaning “that,” introducing a content clause. It is not the infinitive marker “to.” The conjunction að by itself does not force the subjunctive; it’s the meaning of the main verb (vona “to hope”) that selects the subjunctive in the subordinate clause.

Why is it veðrið (definite “the weather”)? Could I say just veður?

Icelandic typically uses the definite form when referring to “the weather” in a given time/place, much like English says “the weather.” So veðrið is normal. You will see the bare noun veður in set phrases and existential statements like Verður gott veður á morgun? (“Will there be good weather tomorrow?”), or in compounds (e.g., veðurspá “weather forecast”). In this sentence, where a specific stretch of weather is in mind, veðrið is the idiomatic choice.

Why is the adjective gott and not góður or góð? Shouldn’t a definite noun take a “definite” adjective?

Veðrið is neuter singular nominative, so the adjective must agree: neuter singular is gott (masc: góður, fem: góð, neut: gott).
The adjective is predicative (after a copular verb like vera/verða), and predicative adjectives generally use the strong endings even when the noun is definite. The weak (“definite”) adjective endings appear chiefly with attributive adjectives before a definite noun (e.g., hinn góði dagur). So veðrið er/verði gott is correct.

Can I use sé (from vera) instead of verði? What’s the difference?

sé is the present subjunctive of vera (“to be”). Ég/við vonum að veðrið sé gott means “We hope the weather is good (now/at that time).”
verði (subjunctive of verða, “become/will be”) usually points to a future or upcoming state: “We hope the weather will be good.” So choose sé for a current state you hope is true, and verði for a hoped‑for future state.

I’ve heard Icelandic has no future tense. Then how does this sentence mean “will be good”?

Correct—there’s no dedicated future tense. Icelandic expresses future meaning with:

Present tense plus context (as here with the subjunctive): að veðrið verði gott.
The auxiliary munu: að veðrið muni verða gott (acceptable, but heavier after a verb of hoping).
Verbs like ætla or vera að fara að in other contexts. In your sentence, the subjunctive naturally carries a future‑looking hope.

Could I say Við vonum að veðrið muni verða gott?

Grammatically yes, and you’ll encounter it. Stylistically, many speakers prefer not to stack both “hope” and a future auxiliary; Við vonum að veðrið verði gott is leaner and more idiomatic. Using muni verða tends to add a “prediction” flavor on top of “hope.”

What’s the word order inside the að‑clause? Could I say að verði veðrið gott?

In clauses introduced by að, the normal order is Subject–Verb–(rest): að veðrið verði gott. Icelandic main clauses are often verb‑second (V2), but after að you do not apply V2. Fronting the verb (að verði veðrið gott) would sound marked or poetic, not neutral speech.

Does vona always trigger the subjunctive? Are there alternatives like vonumst til or vonandi?

After vona with a content clause, the subjunctive is the norm: Ég vona að hann komi; Við vonum að veðrið verði gott.
You’ll see the indicative occasionally in casual speech/writing, but it’s less idiomatic after vona.
vonast til (þess) að… is another common way to say “hope (for) that …”: Við vonumst til (þess) að veðrið verði gott. It’s stylistically fine; it can feel a touch more formal or “wishful.”
vonandi is an adverb meaning “hopefully”: Vonandi verður veðrið gott. It takes the indicative because there’s no governing verb selecting the subjunctive.

How do I negate the subordinate clause? For example, “We hope the weather won’t be good.”

Place ekki before the adjective or past participle, after the verb:

Við vonum að veðrið verði ekki gott. (“… won’t be good.”) If you want “not bad” instead of “won’t be good,” say:
Við vonum að veðrið verði ekki slæmt.

Do I need a comma before að?

No comma is required in modern Icelandic before að when it introduces an integrated content clause: Við vonum að veðrið verði gott. Some older or more formal styles used more commas, but current standard usage would not put one here.

Can I rephrase with “there will be good weather,” like English “We hope there’ll be good weather”?

Yes. A very natural Icelandic variant is:

Við vonum að verði gott veður (á morgun). Here you use existential verði with the indefinite veður (“there will be good weather”), instead of the definite veðrið as the subject.

What forms of vona and verða should I recognize here?

vona (present indicative): ég vona, þú vonar, hann/hún/það vonar, við vonum, þið vonið, þeir/þær/þau vona.
verða (present indicative): ég verð, þú verður, hann/hún/það verður, við verðum, þið verðið, þeir/þær/þau verða.
verða (present subjunctive): ég verði, þú verðir, hann/hún/það verði, við verðum, þið verðið, þeir/þær/þau verði. In our sentence, verði is 3rd person singular present subjunctive.

What case is veðrið in, and how does the noun veður decline?

veðrið is nominative singular definite, functioning as the subject of the subordinate clause.
Basic singular forms of veður (neuter, mostly singular only):
- Nominative: veður; Definite: veðrið
- Accusative: veður; Definite: veðrið
- Dative: veðri; Definite: veðrinu
- Genitive: veðurs; Definite: veðursins

Any quick pronunciation tips for the sentence?

Stress the first syllable of each word.
ð (as in við, veðrið, verði) is the voiced “th” sound (like in “this”). þ (not present here) is the voiceless “th” (like “think”).
tt in gott is preaspirated; it sounds a bit like an “h” before “t” (approx. “goht”).
Rough guide: Við vonum að veðrið verði gott ≈ “Vith VOH-num ath VEH-thrith VER-thi goht.”