1. I didn't know that you were in the hospital yesterday, so I didn't visit you.
(QK)
Nếu tôi biết rằng...., tôi đã tới thăm bạn (trái QK) – L3
→
If I had known that you were in the hospital yesterday,  I woudld have visited you.
2. My mother is very busy, so she can't help me now. (Hiện tại)
Nếu mẹ tôi k bận, bà ấy có thể giúp tôi (trái Hiện tại) - L2
→ If my mother....weren't busy, she could help me now
4. Lan didn't call me because she didn't know my phone number. (QKĐ)
Nếu Lan biết sdt của tôi, cô ấy sẽ gọi cho tôi (trái QK) – L3
If if she had know my phone number, she would have called me