1. has sung- sat
2 went - had sold
3. have learnt - was
4.was walking - met
5. was living

1.(1) Had sung (2) sat ( Vì QKHT + before + QKĐ )
2. (1) Went (2) Had sold ( QKĐ + After + QKHT )
3. (1) Have learnt/ Have learned (2) Was ( HTHT + since + QKĐ )
4. (1) Was walking (2) met ( Đây là dạng quá khứ tiếp diễn : diễn tả 1 hành động đang diễn ra ở quá khứ thì bị 1 hành động khác chen ngang vào,hành động đang diễn ra chia ở thì QKTD,còn hành động chen ngang chia ở thì QKĐ )
5.Were living

1. has sung- sat
2 went - had sold
3. have learnt - was
4.was walking - met
5. was living