Please compare can and could:
  • I can go out tonight (= there is nothing to stop me)
  • I can't go out tonight.
  • I could go out tonight (=but I'm not very keen)
  • I couldn't go out last night (=I wasn't able)
Can or may -> I go out tonight? (=do you allow me to go out?)

Please compare will/won't and would:
  • I think I will go out tonight
  • I promise I won't go out tonight
  • I would go out tonight but I've got too much to do
  • I promised I wouldn't go out
Please compare shallshould or ought to:
  • Shall I go out tonight? (=do you think it is a good idea?)
  • I should / ought to go out tonight (=it would be a good thing to do)
Please compare must and needn't:
  • I must go out tonight (=it is necessary)
  • I mustn't go out tonight (=it is necessary that I do not go out)
  • I needn't go out tonight (=it is not necessary that I go out)
Please compare could have... and would have... etc.:
  • I could have gone out last night but I decided to stay at home
  • I would have gone out last night bit I had too much to do
  • I should / ought to have gone out last night. I'm sorry I didn't
  • I needn't have gone out last night (=I went out but it was not necessary)