necessity
Formal explanation
Need to is the common form. Do not need to / need not means it is not necessary.
Need to is the common form. Do not need to / need not means it is not necessary.
1. You ______ turn the volume up.
2. The file is small. You ______ worry.
3. We ______ make a folder.
4. Optional extra. You ______ do it.
5. Correct: