Jack may be upset. I can't really tell if he is annoyed or tired.
2. Jack may have been upset. I couldn't really tell if he was annoyed or tired.
3. Jack may get upset if you don't tell him the truth.
Jack may not be upset. Perhaps he is tired.
2. Jack may not have been upset. Perhaps he was tired.
3. Jack may not get upset, even if you tell him the truth