If it is a listening exercise only je is accepted as that is what is being said.
A grammatical gender is not the same as the gender that persons, animals and plants have. :) (If you want to know which gender 'melk' has, it's a feminine word!)
You have to learn by heart. (And you can look it up in the dictionary.) But:
- All plurals are 'de' words.
- All diminutives are 'het' words, except for the plural ones, which are 'de' words.
There are a few 'rules' but there are so many exceptions... But, when in doubt, use 'de'. About 75% of the nouns are 'de-woorden'. ;)
What is the different between "hebt" and "heeft" and "hebben". And when we have to use "je" and "jij"?
Je en jij is in principle the same, and are both correct in the sentence showed in Duolingo. However, 'jij' is a slightly more 'emphasized' version. For example the sentence 'jij hebt de melk' would rather be used in a situation like: person 1: Wil je me de melk geven? person 2: Jij hebt de melk daar bij je staan.
Or an example with opening the door: Person 1: Wil je de deur openen? person 2: Jij hebt de sleutels.
Here 'je' would be incorrect. So 'je' is used to point out in a neutral way that the other person has the milk/keys/whatever. 'Jij' is used to really point out that the other person has the milk/keys/etc.
In the sentence shown in Duoling both 'je' and 'jij' are correct. In real conversation it depends on the situation which is used.