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Hi

I have read so much about modals and this is still a grey area of grammar to me.

Need is a semi-modal, which combines the qualities of auxiliaries and 'normal' verbs. OK.

Examples:

I needn't have gone to work (was unnecessary, still I did)

I did not need to go to work (was unnecessary, not inferable if I did)

Semantic difference between need as a modal and ordinary verb

Sometimes, however I cannot decide which form I should use:

You needn't be 18 to be allowed to enter this club.

You do not need to be allowed...

Difference? IMO, no.

Major changes need be implemented.

Major changes need to be implemented.

diff?

Thanks for possible suggestions.
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I needn't have gone to work (was unnecessary, still I did)
I did not need to go to work (was unnecessary, not inferable if I did) -- These are different because you have set them in different tenses. The appropriate comparison is between I needn't go to work and I did not need to go to work-- which have the same meaning.

You needn't be 18 to be allowed to enter this club.
You do not need to be 18 to be allowed...
Difference? IMO, no.-- I agree

(X) Major changes need be implemented.
Major changes need to be implemented. -- The semiauxiliary does not normally appear in affirmative sentences-- only negative (They needn't be) and interrogative (Need they be?).

I cannot offhand think of cases where the meanings are different; perhaps another member can.
Comments  
"The semiauxiliary does not normally appear in affirmative sentences-- only negative (They needn't be) and interrogative (Need they be?)."

good to know, thanks.