why seule and not seul?
Adjectives match the noun's gender and number. In this case, "fille" is feminine, so you must use the feminine form "seule".
When I typed "La fille est seul" without the e at the end I started to panic because I thought it was going to be wrong. But then duolingo still accepted my answer. Hmm, is my answer still grammatically correct even if I forgot the e at the end?
No, it's because You made only one mistake. Duolingo let You make one mistake, but is always the information that You almost wrote the sentence right.
but this mistake can be of course 'a little' like this, only one letter.
When do I use seule/seul?
isn't fille both girl and daughter?