Why is derrière in the feminine form?
'Derrière' is a preposition, like 'sur', 'sous', 'dans', 'entre', 'devant', ... It doesn't have a masculine and feminine form – it's invariable.
I put.. il y a un animal caché en arrière de cet arbre. It was marked wrong. What is the diff between derrière and en arrière de ?