Why do we use ''första'' and not först?
I think it has to do with adjectives being in the plural/definite form after a possessive pronoun, but that's just off the top of my head.
After much research this definitely seems to be the case, thank you! :)
Can hit not be used here?
No, it can't. "Hit" is used if there is a movement, e.g. Will you come here? = Kommer du hit?
I don't understand why it is vecka and not veckan... Shouldn't it be the definite firm after min and an ordinal number ?
Why is it Det and not den if it is veckan and not veckat?
The det är at the beginning of a sentence is a set, unchangeable construction when it is used to mean "it is..." in an impersonal sense.