Okay, I know it should be in the nominative, now I need to know why. Is it because "To" is in the nominative, and, since it is describing "pies," "pies" needs to agree with the adjective?
"To" doesn't serve as a pronoun here, it takes the role of the subject - for which the English equivalent is either this or these. Therefore it's always 'to' no matter whether we're talking about masculine, feminine, neuter or plural. And such basic constructions "This is X" and "These are Y" simply look like this every time.
To jest -nominative singular-
To są -nominative plural-
"To" is functioning as a pronoun - it's replacing a noun in the subject. What you're talking about is that "to" doesn't decline for gender or number before a copula. Is that also true in tenses other than present?
Someone suggested elsewhere that when the instrumental case is used with być, it is like saying someone or something is being in the manner of something else. It is rather a redefinition than a mere definition.
On the other hand, we use To jest to introduce something for the first time. It is not a redefinition, so we can make sense that the instrumental case does not apply.
No. First of all, this is an "This is Y" type of sentences, so there's no he/she/it here. Secondly, if I were really to use a similar construction (let's say that I'm talking about Hektor, and my interlocutor thinks that Hektor is a person, but in fact Hektor was my friend's dog), I would use "on" or "ona" depending on the dog's gender, and not neuter "ono".
Hmm. I guess for such things as answering the phone/making a phone call and introducing yourself you could use "Mówi pies", "Z tej strony pies/Pies z tej strony", "Przy telefonie pies" or something like that. But not "To jest pies". No one in Poland would say about themselves "To jest [name]", because that's referring to something 3rd, not yourself.