You could say "She is a child." which would be "Zij is een kind." or "He is a child." which would be "Hij is een kind.", but there are times when you might say "It is a child." and that would be "Het is een kind." and from that we do not know the gender and maybe the speaker is too far from the child to see the gender either.
That would be "Dit is een kind." http://webtranslation.paralink.com/translator/default.asp
Een means both "a," "an" and "one." The difference is really in the pronunciation. You'd say Een ( pronounced as Ain) when you mean one of something, i.e. One child, Een kind. When you mean to use the English A or An, in Dutch you'd still use Een, except you pronounce it like Uh(n). Hope this clarifies things.
Het and De both mean The, i.e. Het meisje, De jongen (the girl, the boy). Het can also be used for It, but it depends on the sentence it is used in. For example, Het is koud, It is cold. As for Zij/Ze, in singular it means She, in plural it means They, i.e. Zij drinkt (she drinks) and Zij drinken (they drink).
- Het can be it (if it is the subject) and the
- Zij singular = she, e.g. zij is - Zij plural = they e.g. zij zijn
- Deutsch = German, not Dutch
The app is made to learn by making mistakes and finding patterns yourself and by using external information if needed.
You can also have a look here: https://www.duolingo.com/comment/3732817