SHOULD - OUGHT-TO - NEED

SHOULD_OUGHT-TO

Should and ought to are interchangeable for advice and conclusions :

• You should / you ought to write to your grandmother.
• He has revised all day; he should / he ought to be ready for his exam.

Should is also used : 

• In hypothetical situations :
  • Should anyone call, please take a message.
  • Should you need any help, just call me.
  • If I were you, I should leave.
    (In this last example, 'would' is often used today.)
• With 'imagine', 'say' and 'think' to give tentative opinions :
  • I should think the journey will take about two hours.

NEED

Need expresses necessity.

• In summer we need to drink more often (= it is necessary for us to drink often.)
• You needn't / you don't need to take notes.  You'll get a summary.
  (=it is not necessary to take notes.)
• Need I / Do I need to do this? (=Is it necessary for me to do this?).

NEEDN'T HAVE / DIDN'T NEED TO

• Needn't have is used to refer to an action which was performed but later turned out to be unnecessary.
  • We needn't have rushed to the airport; the flight was delayed.
  • He needn't have taken a sweater; the weather was warm.
• Didn't need to is used to refer to an action in the past which was not performed
  because there was no need for it. It was not necessary.
  • We didn't need to show our passports - ID cards were accepted.