1 had ( QKĐ + while +QKTD, cũng có trường hợp while + QKĐ nhưng hiếm khi xảy ra)
2 were arriving ( QKTD +when +QKĐ)
3 was starting ( do nghĩa)
4 were coming (when +QKĐ)
5 wasn't going (yesterday)
6 changed (since)
7 covering ( used to v)
8 use (for)
9 play ( giới từ + ving)
10 blows(now)
11 surfing (used to v)