Before attempting lesson 11 make sure you read this first.

I started learning ancient Greek using this Athenaze course and thought it easy. Till I hit lesson 11 and got culture shock:
  • The "Preview of New Verb Forms" was bad enough, but lesson 11 was worse.
  • Lesson 12 was slightly more amenable,
  • but it wasn't till meeting the imperfect in lesson 13 that the fog lifted.
That's why I suggest you follow my advice and go over 3 pages I compiled about the imperfect, before trying to wade through the overwhelming amount of theory thrown at us in lesson 11.
Mind you, I really like this book and think it is the best introduction to ancient Greek by far, but, as all books, it has some weak spots. Lesson 11 is one of them. I finally survived and I hope so will you.

Imperfect Verb Forms:  1. verbs starting with a consonant.
Imperfect Verb Forms:  2. verbs starting with a vowel (or diphthong)
Imperfect Verb Forms:  3. middle forms

Knowing how the imperfect works will make learning the 2nd aorist, the tense taught in lesson 11, much easier. Why?
  1. Because all verbs have imperfect forms, while only some have 2nd aorist
  2. Because the imperfect forms introduce 2 important concepts:
    past tense augment
    secondary endings
  3. Because the imperfect does NOT imply change of stem
  4. Because the imperfect doesn't complicate matters by having special infinitive, imperative, subjunctive, optative and participle forms. Never heard of optative (from Latin optare = to wish)? You will, a little later on. To make your life complete.