Although you may want to include Lessons in your sideboard if you’re playing with cards that instruct you to learn, Lesson cards can be included in your main deck like other instant or sorcery cards.

Lesson is a spell subtype found on some instant and sorcery cards in the Strixhaven set. The Lesson subtype has no special rules associated with it.

You choose Frog or Octopus as Mercurial Transformation resolves, not when you cast it.