Final answer:
The use of 'needn't' (need not) followed by 'to' is incorrect when 'to' is intended as part of an infinitive verb; however 'to' can be used if it is a preposition in its own right and not part of the infinitive form of the verb that follows 'needn't'.
Step-by-step explanation:
The use of 'needn't' followed by 'to' depends on whether the 'to' is part of the infinitive form of the verb that follows or simply a preposition. 'Needn't' is a contraction of 'need not' and when it is followed immediately by a verb, that verb is typically in the infinitive form without 'to', which means 'to' is not used.
For example, 'You needn't worry' is correct, while 'You needn't to worry' is incorrect. However, if 'to' is a preposition that's part of a prepositional phrase and not part of the infinitive, it can be used. For instance, 'You needn't walk to the store if you don't want to,' in which the first 'to' is a preposition related to 'walk', and the second 'to' is part of the infinitive verb 'want to'.