Why not ubi panem est?
You can't use panem here (an accusative) since there is no direct object in this sentence. When you use esse (to be) there will be nominatives on both sides.
Does that help?
"To be" is a special verb that never takes the accusative.
It's not considered as a direct action on something (=accusative)
It's called a "copula" in linguistics.
The bread is not the object of this verb, the verb 'to be' describes things as they are, nothing is being done to it so it stays unchanged (the nominative case) as 'panis.'
If it was the object of a verb, like "I have bread", "panem habeo."
Everyone asks "where's the bread", no one ever asks "how's the bread". ;_;
Because no one answered their question. How can they answer your question if they can't find the bread in the first place.
Ubi panis est?
Quomodo panis est?
Isn't "cur" = "why"?
And then when you find the bread: "ecce panis". Which would make a great name for a bread company!
...wait a minute...