Why do I have to use the definite article in this sentence? Could somebody explain it to me?
As always there is an exception ...mio, tuo, suo, nostro, and vostro.. do not have the definite article in the singular. Loro is the exception and takes the article, so il loro fratello is correct http://italian.about.com/od/grammar/a/italian-possessive-adjectives.htm (about halfway down)
I think it shouldn't be mandatory to use the article. Anyway, with "il" it sounds better, I cannot explain you why. If you have more examples you don't understand, please list them and ask me again, we will try to find reasons behind what is more common in Italian.
Report it, anyway, without "il" is as correct as with it.
I know this a free app, but it would be nice if the voice was more clear. I miss a lot of these because i cant understand her...
I have tried headphones and find it makes no difference. Some sounds are just to quiet to hear.