Learn Danish: Beslutningen var helt forhastet. - The decision was completely hasty.

You could say Du skulle ikke have truffet en forhastet beslutning. Here you see the pattern for unfulfilled past obligations: past tense modal (skulle) + have + past participle (truffet). Note that Danish uses the verb at træffe (to make/take) with decisions, rather than "at lave".

Why is it helt with a -t instead of just hel?

Here, helt acts as an adverb meaning "completely", modifying the adjective forhastet (hasty). In Danish, when you use an adjective as an adverb to describe how something is done or to what degree, you add a -t to the end, which makes it look identical to the neuter form of the adjective.

Forhastet looks like a past participle. Does it change depending on the noun?

Yes, it acts as an adjective here. Because beslutningen is a common gender (en) noun, it takes the standard form forhastet. If you were describing a neuter noun like et valg (a choice), it would still be et forhastet valg. However, in the plural, you must add an -e: forhastede beslutninger (hasty decisions).

Can I use forhastet to describe a person who is in a hurry?