What is the difference between the two sentences in each of the three cases?
1A. I should like to have sat down.
1B. I should have liked to sit down.
2A. He acts as if he knows the subject.
2B. He acts as if he knew the subject.
3A. He felt better when she gave him medicine.
3B. He was feeling better when she gave him medicine.
The first pair confounds me the most.