"He sees that child."
Translation:On widzi tamto dziecko.
Why is tego wrong? I think im just having a mental block or i dont understand when to use tego.
Take a look here, it describes the this/that issue quite thoroughly: https://www.clozemaster.com/blog/polish-demonstrative-pronouns/
As this specific sentence is considered: "widzieć" takes Accusative, and "tego" is indeed an Accusative form, but for masculine animate nouns. For example "On widzi tego chłopca" means "He sees this boy".
"dziecko", however, is neuter. And for neuter singular, the Accusative form is identical to the Nominative one, so it is "to".
Or rather, in fact, it's "tamto". The English sentence uses "that", for which the direct equivalent is "tamto" and "to" is just an acceptable approximation.