Question

3. It was not until he got home      he remembered leaving the door key where he had played basketball with his classmates.

Answer 1

3. that强调句中被强调的部分是milil引导的时间状语从句。当until部分被强调时，主句中的否定词not要移到until前，形式为"It is/was not until - +that从句"。