In Swedish, the genders are not masculine or feminine."En" words are called common (or uter), and "ett" words are called neuter (or neutrum). It is difficult to tell a word's gender based on how it is spelled or how it sounds, so you should memorize the genders when you learn the word. It isn't that hard, though, because the gender will become clear once you make the word definite. If a word ends in -en in the singular definite form, it is an "en" word. If a word ends in -et in the singular definite form, it is an "ett" word. The majority of the words are "en" words, but there are still many, many, many "ett" words (so if you absolutely have to guess, pick "en"). Hope my explanation helped!

It's totally ok to say both Hon är en flicka and Det är en flicka in Swedish so in the course, we'll stick to translating 'She's a girl' into Hon är en flicka and 'It/That's a girl' into Det är en flicka. There's a bit of difference in perspective.

If you got this in the food lesson, it's probably because of a bug or possibly A/B test that's been going on for a while – sentences seem to be moving around on their own accord.

The machine tries to match your input to the closest accepted answer. tjej is a slightly more colloquial, but very common word meaning girl (and girlfriend). It's taught a little later in the course but since it's in the accepted answers, it can get shown here too.