$\text{11.}$ was visiting

$\text{→}$ At this time last month: thời gian xác định ở QK ⇒ dùng QKTD

$\text{12.}$ repainted

$\text{→}$ All last week ⇒ dùng QKD

$\text{13.}$ met - told

$\text{14.}$ was skipping - was playing

$\text{→}$ While ⇒ thường dùng thì QKTD ở cả 2 vế câu

$\text{15.}$ were making

$\text{→}$ 3 and 5 yesterday afternoon: có thời gian xác định ở QK ⇒ dùng QKTD

$\text{16.}$ was repairing - felt

$\text{→}$ Khi tôi đang sửa mái nhà, tôi ngã khỏi cái thang.

$\text{17.}$ was watering - came

$\text{→}$ Anh ấy đang tưới rau khi tôi đến nhà của anh ấy.

$\text{18.}$ were having - rang

$\text{→}$ Khi chúng tôi đang ăn tối, điện thoại kêu.

$\text{19.}$ saw - was sitting

$\text{→}$ Khi tôi nhìn thấy anh ấy, anh ấy đang ngồi trong quán cà phê.

$\text{20.}$ was slicing - cut

$\text{→}$ Trong khi cô ấy đang thái thịt bò thì cô ấy cắt phải bản thân.

$\text{21.}$ were waiting

$\text{→}$ From 4 to 6 yesterday afternoon: thời gian xác định ở QK ⇒ dùng QKTD

Câu 11: was visiting

Câu 12: repainted

Câu 13: met - told

Câu 14: was skipping - was playing

Câu 15: were making

Câu 16: was repairing - fell

Câu 17: was watering - came

Câu 18: were having - rang

Câu 19: saw - was sitting

Câu 20: was slicing - cut

From 4 to 6 yesterday afternoon, we (wait) were waiting for our teacher at the station 

                                 Chúc bạn học tốt 

                        Nhớ vote 5 sao và ctlhn nha