Originally posted by: Kai_E. https://forum.duolingo.com/comment/3734833
Negation
In Dutch, you use the words "niet" and "geen" when negating things.
”geen”
"geen" is used to negate a noun that, if not negated, would be preceded by "een".
Is dat een koe? - Nee, dat is geen koe.
(Is that a cow? - No, that is not a cow.)
It can also negate nouns that aren't preceded by any article, like some nouns in the plural and uncountable nouns.
Hebben jullie boeken? - Nee, we hebben geen boeken.
+ (Do you have books? - No, we don't have books.)
”niet”
"niet" is essentially used in all other situations:
to negate verbs, thoughts, adjectives and any other sentence elements that aren't nouns:
Ik ren niet. - I do not run.
Ik niet. - Not me.
Hij is niet zo oud. - He is not that old.
to negate nouns preceded by a definite article or possessive pronoun:
Nee, dat is niet mijn boek. - No, that is not my book.
Nee, hij was niet de burgemeester. - No, he was not the mayor.
Where does "niet" go in a sentence?
It depends on what you are negating. If you're trying to negate something particular like an adverb or adjective, then it's best to put "niet" right before it.
Mijn rok is niet geel. - My skirt is not yellow.
Ik eet niet altijd vis. - I do not always eat fish.
In most other cases, "niet" comes after the "middle part" of the sentence - where you usually have the time, manner and place.
Ik heb hem gisteren (time) niet gezien. - I did not see him yesterday.
Ik heb door het lawaai (manner) niet kunnen slapen. - I could not sleep due to the noise.
Put "niet" here, and you will likely be right.
Despite the "place" usually being in the middle part of a sentence, "niet" usually comes before it when it indicates a direction.
Wij gaan niet naar huis. - We are not going home.
However, if you put "niet" in front of the time, manner and place, then you are stressing that it was not then that I did it (but later), that it was not there that I did it (but here), or I didn't do it like that (but like this).
Hij gaat niet vandaag naar de maan, maar morgen. - He is not going to the moon today, but tomorrow.
Wij gaan niet met jullie, maar met hen. - We are not going with you, but with them.
Exercises
Here are some exercises that may help you with negation in Dutch:
N 
C1 
B2 
B1 
A1