"That is an egg."

Translation: To jest jajko.

[Pampelius]

Why jajko and not... jajkiem or whatever the instumental would be

[emitoska]

My Assimil Polish course states that with "to" you can usually omit "jest", thus I wrote "To jajko". This was marked as incorrect, and I've just grasped that this could also mean "this egg".
But still, in a sentence like "To nie warzywo, a to jajko" it should be possible to omit "jest" both times, right?

[doro.tab]

"To jajko" should be correct in here. But in the sentence "to nie warzywo, a to jajko" it's not correct. "A to" suggests that you are switching to talk about different object now, so the whole sentence sounds bit like "this is not a vegetable and that egg". It should be "to nie warzywo, a jajko" or "to nie warzywo, ale jajko" (both mean "It's not a vegetable, but egg").

[emitoska]

Got it! Dzięki :)

[PonyDesu]

"To nie warzywo, to jajko" or "To nie warzywo, a jajko".