Lesson 21 introduces the equivalent of "Which" or "What kind" that are used to identify or characterize different nouns.
Also, lesson 21introduces the use of clauses as objects in sentences. For that, as in English, different question words are used as a link between the predicate and the clause. To do well with exercises for this lesson, students should be fluent with interrogative words (question words) that have been introduced from lesson 1 until this point
Also, lesson 21 introduces the particle ..."Sheh"