Learn Swedish: Alla älskade museet. - Everyone loved the museum.

When alla is used all by itself as a pronoun, it translates to 'everyone' or 'everybody'. If it were describing a noun, like in alla museer (all museums) or alla människor (all people), it would just mean 'all'.

How do we know älskade is in the past tense?

It has the classic -de ending of a regular Group 1 verb. The present tense is älskar (love or loves), and replacing the -r with -de gives us the past tense älskade (loved).

Why is 'the museum' spelled museet? Shouldn't it be 'museumet'?

Ett museum is an exception because of its foreign ending. For ett words ending in -um, you drop the -um before adding the definite suffix -et. That is how we get museet instead of 'museumet'.