The comments from the sentence about the museum not being open on Mondays indicated that har öppet was the better translation. Is the distinction habitual (i.e., the museum is never open on Mondays) vs. point in time (i.e., it's 9:55 and the museum doesn't open until 10)? Tack!
Neuter nouns ending in -um tend to work differently, and behave differently also depending on whether the letter before the suffix is a vowel or a consonant.
museum follows the basic pattern of polysyllabic nouns ending in -eum and -ium, namely that it drops the -um and then behaves like this:
- singular definite: -et, museet
- plural indefinite: -er, museer
- plural definite: -erna, museerna
But there's at least one exception to that as well... so it's complicated.