The natural order hypothesis says grammar features are acquired:

1. The hypothesis says all learners acquire a language in roughly the same order. 2. This order does not depend on how easy a feature is to teach; the English third-person "-s" is easy to teach but acquired late. 3. So the order is not set by the textbook or the teacher. 4. The correct choice is a similar order for every learner. **Why not the others:** A textbook, a teacher or ease of teaching cannot fix the order, because the order is the same for all learners whatever the teaching. _Source: Wikipedia, "Input hypothesis", section "Natural order hypothesis" (revision 1380165607)_