I want to point out that while Swedish has this construction that English doesn't have, there's also an English construction that cannot be used in Swedish:
The Museum opens from 10am to 5.30pm every Tuesday – Sunday
We can't say that with the present tense of the verb öppnar in Swedish. It would have to be museet har öppet or museet är öppet. In Swedish, when museet öppnar, that is something that happens at a point in time. It cannot be stretched out in time from 10am to 5.30.
So you can say Museet öppnar klockan 10 = The museum opens at 10, but that only refers to the point in time when it opens. After that, it is open (det är öppet or det har öppet).
so would you say something along the lines of 'det är oppet från 10 tills 5' or 'det har oppet från 10 tills 5'?
Yes, both those work, except I'd say från 10 till 5 – I think people disagree about whether "tills" is wrong in this context or not, but till is overwhelmingly more common and also what would traditionally be considered correct (tills is historically a contraction of till dess so it basically means 'until [that]'.) – in many contexts both are used, but between two figures like this, till is preferred by most.
I incorrectly translated this as "They have opened on Sundays". Is that necessarily wrong? If so, what would that be in Swedish? It's the present perfect tense isn't it?
It would be De har öppnat på söndagar in Swedish, though I think that sentence would need an object to be totally intelligible in either language. har öppnat and have opened is the present perfect, yes.
It is a fine sentence in English, where you could add "...since the Easter holidays." or similar.
Why is "öppet" with a -t? And can you say "De är öppet" or must it be "har öppet"?
"Ha öppet" is the common construction to say that they're open.
Although less common, you can also construct it like de är öppna, but then öppna has to plural (öppen-> öppna).
It's just one of those things that are different; you accept them and learn them.
What is the word type of "öppet" in this case? Adverb? If so, then what's the meaning of the "ha" word here? Like, the logical explanation why we use "ha" and not "är", for example. Other examples of using "ha öppet"? När använder vi det?
I translated it as "They have open on Sundays." It was marked incorrect, but here in the Pennsylvania German part of the state that term "have open on Sundays" (or weekends or holidays, etc.) is quite common. I don't know if that's true anywhere else.
Might originate in German. We also say "sie habe offen" wich is literally "they have open"
Interesting. I haven't heard that out west (Colorado/Wyoming/South Dakota/Montana).
No, öppen needs to have the same gender and number as its main word, so that would be de är öppna. That's a possible sentence, but note that we don't use it the same way you can say They're open in English – for instance to say that a store is open. To say that, we must say De har öppet. If you are speaking about several objects of people though, de är öppna works. Like, question: Are the windows open or closed? Answer: De är öppna.
Is it "oppet" because when you refer to "det" without reference, it's always "det" not den? (does that make sense?)
Att öppna, öppnar, öppnade, har öppnat - is the verb (in infinitive, present, past and perfect present tenses respectively). Verbs do not change with singular/definite/plural.
Öppen is an adjective. Adjectives do change in the manner you describe. Öppen, öppet, öppna
eg. En word: Dörren är öppen
Ett word: Barnet är öppet (open can also be used to describe a personal characteristic much like in English.)
Plural: Dörrar är öppna.
Definite (double definite form): Den öppna dörren