Re: "Allegory" in simple terms...
A metaphor is short: He is a pig.
An extended metaphor can be a bit longer: Too many cooks were involved in this project and not surprisingly we ended up with an over-seasoned, gloppy soup instead of an elegant main course.
An allegory is an entire story. You can read an allegory as a simple story. You can read Animal Farm and think it's about animals.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.