They lived happy ever after.

What is the mistake and how can I explain it?

You used the adjective, "happy," when you should have used the adverb, "happily."

The adverb tells how they lived, so modifies the verb, "lived."

Thank you!

You're welcome.

The mistake in the sentence is the word "happy". It should be "happily" instead.

To explain the mistake, you can point out that "happy" is an adjective, which describes a noun. In this case, "ever after" is an adverbial phrase modifying the verb "lived". Therefore, an adverb is needed to describe how they lived. The correct form should be "happily ever after" since "happily" is an adverb that describes the manner in which they lived.