It is necessary for you to work hard.
You must (or have to)work hard.
1.It will be necessary for you to see a doctor.
2.Is it necessary for you to make so much noise?
3.She said it would be necessary for us to stay here.
4.It is necessary for me to have some help.
5.It was necessary for him to go out last night.
You have to(must) see a doctor.
Do you have to make so much noise?
She said we had to(must) stay here.
I have to (must) have some help.
He had to(must) go out last night.
第二句我觉得不要用must来替换了,have to比较好.