How are we gonna know if it is a female or a male if it only says the cat is mine? It should be el gato es mío.
el gato es mío.
Both work in this case: "El gato es mío" and "La gata es mía". What does not work is "La gata es mío", since that would be mixing the genders.
Why "La gata es mio" is not a correct translation for "The cat is mine"
Since that is a female cat, it would be "La gata es mía" (not "mío").
How come it depends on the gender of the cat instead of the gender of the speaker?
That's how it is for all adjectives in Spanish.
How do you know when to use the accent on mio/mia? Sometimes there will be an accent and sometimes there won't be, but I can't seem to find a pattern that will tell me whether to use an accent or not.
mío/mía always have an accent in the i.