Pets are not allowed in this supermarket. You must not bring your pets here.It's not necessary for him to come round before 5 pm. He doesn't have to come round before 5 pm.It's your moral obligation to try and protect the environment. You must try and protect the environment.I'd better learn for the test so I don't fail this time. I must study for the test so I don't fail this time.It's not necessary for you to post this ad for me on the internet. You needn't post this ad for me on the internet.It's your duty to be more careful. You ought to be more careful.