'Show me' is the same even if 'to' is not added.
RU: Pokaży mnie eto!
Why does it not allow"show me"
Without "this"? Frankly... I can see the point. I guess it won't hurt, added.
Why is it the perfective here? Why not "pokazuj mi to"?
Because to is something specific. Perfective aspect is used in a specific event with a specific setting. Imperfective aspect is used in general, non-specific instances.
Oh that makes more sense now, thanks!
But why "Nie mów tego.", then? Isn't "tego" specific?
My explanation about perfective and imperfective verbs applies generally. It's not a hard rule set in stone for all verbs. Different verbs have different meanings, and mówić is not pokazać
I'd treat as something 'general' because it's a bit like Do not say that, ever! I think I saw a 'rule' that negations take perfective verbs, but I'm pretty sure there are exceptions from that.