"My younger sister runs every night."
It's not less correct. It's similar to how native Spanish speakers drop off tge 'Yo' in many sentences such as "soy de Taiwan" - as opposed to “Yo soy de Taiwan”.
Colloquially, we in Mandarin when saying “我妹妹” means that you must be talking about “my own little sister”. Hence the lack of a need for “我的”.
@ApolloK the way you argue reflects not only on this particular excercise The problem with Duolingo is that sometimes they accept both and other times they dont accept one but the other. The way I deal with it is that I don't insist in what should accepted but embrace the opportunity to practise one more time. In the end, I benefit from DLs reluctance to do the detailled work. Keep in mind, its free.
My opinion has been that Mother father, little sister and to a lesser extent friend all sound a little off with "de". The reasoning was that these people are close relatives and using it makes it less personal (only exception is pets, as much as they feel like a close relative always use "de").
Both are correct, but Duolingo gives you wrong sometimes because they havent added both options as the correct answer.
has the same meaning and are equally good sentences.
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