Have to, Don't have to

a) We don't have to read that bool, Liam?
b) Oh, Then which book do we have to read?
a) We have to read this book and answer these Questions.

a) The customer service staff are so busy.
    They have to answers so many phone calls!
b) Yeah, but at least they can pick when they want to work.
a) You're right. If they choose to work the afternoon shift,
    they don't have to come to work until 1 pm.
b) On the down side (マイナス面は) ,
    sometimes they have to work on the weekends.

a) We have to get a birthday cake for Jake.
    It's his birthday today!
b) Oh, but he wants to lose weight.
    So we have to get some sort of low-fat cake.