noun
BAD : It was then that I felt the necessity to improve my English.
GOOD : It was then that I felt the need to improve my English.
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If something is a necessity , you must have it or do it, or it must happen:
'If you saw the terrible conditions in which these people are living, you would appreciate the necessity to step up foreign aid.' 'For anyone who runs a large business, a computer is an absolute necessity.'
Need has a similar meaning to necessity but it is also used in connection with something that you want to do/have/happen (although it is not essential): 'I began to feel the need for a change of life style.' 'Don't you ever feel the need to do something more creative?'