Yeah, and thus I tried to translate this as "I am no child," but was marked wrong; though it's a perfectly valid sentence in English*, given the way it's presented in German. I assumed it would be the same for Dutch. Is there an expert among us who can shed some light on the subject?
*I am a native speaker of English.
If we take countable nouns and singular nouns, we need 'not a' in many cases so a new word 'geen' with the same negation value as that of 'niet' seems valid. For the rest two, it should be perfectly okay with 'niet' also...for eg. Zij zijn niet goed mensen; Ik heb meer rijst niet
niet (German: nicht) = not nee (German: nein) = no (as in "No! Stop! Please!") geen (German: kein) = no/not a (as in "I am not a doctor" or "I have no son")
Or in grammatical terms, "geen" can be used as an indefinite article and "niet" cannot. In other word, "geen" is the opposite of "een."
*disclaimer, I'm learning Dutch for the first time, but I've known German for years
Use geen with (indefinite) nouns no matter if they're singular or plural. Ik ben geen muis, we zijn geen muizen.