Questions & Answers about Das Kind hat darauf nicht reagiert.
Why is nicht placed right before the past participle reagiert?
Because we are negating the entire action of the sentence (the child didn't react at all). When negating a whole sentence in the perfect tense, nicht gets pushed as far to the end as possible, landing just before the past participle.
What happens to the meaning if I put nicht before darauf instead?
Placing it before darauf (Das Kind hat nicht darauf reagiert) changes the focus. Instead of negating the verb, it specifically negates the object: The child didn't react to that (strongly implying that the child reacted to something else instead).
Why do we use darauf here instead of just translating 'to it' literally?
The verb reagieren (to react) always requires the preposition auf in German. When the object you are reacting to is an abstract idea or a thing ('it'), you must use a da-compound: da + r + auf. If the child were reacting to a person instead of a thing, you would use a regular pronoun, like auf ihn (to him) or auf sie (to her).
Why doesn't the past participle reagiert start with ge-?