How Languages are Learned 4th edition. Nina Spada

      Negative questions may still be a bit too difficult.

      And even though performance on most questions is correct, there is still one more hurdle. When wh- words appear in subordinate clauses or embedded questions, children overgeneralize the inverted form that would be correct for simple questions and produce sentences such as:

       Stage 6

      At this stage, children are able to correctly form all question types, including negative and complex embedded questions.

      Passage through developmental sequences does not always follow a steady uninterrupted path. Children appear to learn new things and then fall back on old patterns when there is added stress in a new situation or when they are using other new elements in their language. But the overall path takes them toward a closer and closer approximation of the language that is spoken around them.