Why is there vand instead of vandet??
Because it's not a specific mass of water (I drink the water), but just that the speaker drinks water in general (I drink water)
why is this drikker and not drikke?
Because it is in present tense, there is no conjugation like English, so present tense always has the "r" on the end (other than in irregular verbs)
In Danish double consonants often determine the pronounciation - usually the speed of certain parts of a word.
If I am not mistaken, "I am drinking water." / "I'm drinking water." should have been marked as correct, and not incorrect.
Why the letter 'i' is spelt in drikker (as in ice) and in spiser (as in pizza)?