Learn Hardcore Icelandic: Ég er viss um að veðrið verði heitt á morgun. - I am sure that the weather will be hot tomorrow.

In Icelandic the phrase vera viss um e-ð literally means “to be sure about something.” You always need um to link viss (sure) with whatever you’re sure of. English has “sure of,” Icelandic has viss um + accusative or + að-clause.

Why is there an að before the clause veðrið verði heitt á morgun?

Icelandic requires að (similar to English “that”) to introduce a finite subordinate clause. After viss um you’re giving the content of what you’re sure about, so you begin with að. In very casual speech learners sometimes drop að, but standard written and spoken Icelandic keeps it.

Why is verði used instead of verður?

Verði is the present-subjunctive form of verða. After verbs expressing certainty, doubt, desire etc., normative grammar often prefers the subjunctive in subordinate clauses. So að veðrið verði heitt (“that the weather be hot”) is the formal choice. Colloquially you’ll also hear að veðrið verður heitt, using the indicative, but verði is safest in writing.

What does the verb verða mean here? I thought it meant “to become.”

Verða has two related uses:
1) As “to become” (change of state).
2) As a future‐tense auxiliary: verður heitt = “will be hot.”
In your sentence it’s acting like English “will be,” predicting tomorrow’s weather.

Why is veður written as veðrið?

Icelandic marks definiteness with a suffixed article. Veður = “weather” (indefinite), veðrið = “the weather” (definite). You always attach -ið (neuter singular) to make it definite.

Why is á morgun placed at the end of the sentence?

Adverbial time expressions in Icelandic commonly sit either at the beginning or end of a clause. Putting á morgun at the end is the most natural, focusing first on your certainty and the clause, then specifying “tomorrow.”

Can I use mun instead of verða to talk about the future?

Yes. You could say:

Ég er viss um að veðrið muni verða heitt á morgun. (muni is the subjunctive of mun)
Informally some speakers even use the indicative mun: …að veðrið mun verða heitt á morgun. Both express the same future meaning, but if you choose mun in a subordinate clause you should use muni for the subjunctive mood.