"They are playing inside."
Translation:Ils jouent dedans.
I wrote 'jouent' because I knew it meant 'play' but I hovered my mouse over the word 'playing' and I saw that it can also mean 'interpretent' (with an accent on the e). So I wrote 'Ils interpretent dedans' and it marked me incorrectly.
Because they don't use the present participle for that, not the way we use the gerund. http://french.about.com/od/grammar/a/presentparticiple.htm