Learn Swedish: Det vore underbart med en stor terrass. - It would be wonderful with a large terrace.

vore means "would be". It is a special form (the past subjunctive) of the verb vara (to be) used for hypothetical situations, wishes, or polite suggestions. You could also say skulle vara here, but vore is very common and sounds a bit more natural for this kind of dreaming.

Why does the adjective end in a T (underbart) if terrass is an en-word?

The adjective underbart is describing the pronoun Det (It) at the start of the sentence, not the terrace itself. Because Det is a neuter word, the adjective gets the neuter -t ending. It basically means "It (the general situation) would be wonderful."

Why does the sentence use med (with) instead of a verb like "to have"?

While you could perfectly well say Det vore underbart att ha en stor terrass (It would be wonderful to have...), using med is a very idiomatic Swedish shortcut. Swedes frequently use med after adjectives describing a situation to mean "having" or "to have".

Why is stor in its base form and not stort?