Why is is "ty psy" and not "ti psy"... aren't dogs animate?
"Ty psy" is accusative, "ti psi" nominative. You use accusative for direct objects. Read the tips and notes provided.
Where are the tips and notes provided? Are you reffering to these comments or so ething more?
If you open duolingo as a web site you'll find a lightbulb button in every lesson. It isn't presented in mobile app though.
If you are speaking about doing something to the dogs, even seeing or looking for them, use "ty psy", the accusative form. If you are just describing/naming the dogs, as in, "the dogs are eating", use the nominative form "ti psi". I think.
Why is "Ten kočka ty psy nevidí" incorrect?
Kočka is feminine.
In the previous sentence the form was tu kočku (já tu kočku nevidím). Why the difference?
Ta kočka was the object, now it is the subject. Check the Tips and notes about the accusative case.