Why can't it be "this sandwich is tasty"?
this sandwich is tasty = הכריך הזה טעים
Ok, so you need the copula?
It's not a copula here. הכריך הזה = this sandwich. זה כריך = this is a sandwich. הזה is located after הכריך just like an adjective in Hebrew goes after the name. הכריך הגדול, הכריך הזה.
You need another ה
Why is it not "this sandwich is tasty"?
This sandwich = הכריך הזה. A slightly different sentence.
zeh caríkh taim