"This is an egg."
Translation:To jest jajko.
Why not "to jajko" - would that be "the egg" or "this egg" instead of "this is an egg"?
It would mean both 'This egg' ('Które jajko chcesz?/which egg do you want?') or 'This is an egg', but rather in colloquial speech. It depends on the context really.
Perhaps to avoid confusion, you should use the sentence only in the first meaning.
why not "To jest jajkiem?" Is this formula appliable only to persons and animals?
Because of "to". It "to" is in the sentence, but does not mean "the" , it changes case after jest to nominative.