a)
- You should come with me next time.; If I were you, I'd just go for it.
- You should have moved a bit more gently.; You shouldn’t have leaned so far, then.
- You could arrive any time after 8:00 in the morning,; You can’t go up alone on your first time.; You can go alone after doing a few flights with an instructor.; Could you do a really low flight? Are you allowed to go up just a little way above the ground?
- Were you allowed to just start without doing any training? I couldn’t go straight up without any training at all.; I wasn’t allowed to go up before having some brief general instructions.
b)
Can and Could can both be used to talk about permission in the present in sentence 1 but Could is a little more tentative.