Have to, don’t have to, and can’t Use have to for general obligations and to show that something is necessary. I have to work shifts. You have to come on time. Use don’t have to to show that something isn’t necessary. You don’t have to wear a suit. Use can’t to show that something is not allowed or isn’t a good idea. You can’t lie in a job interview. Also use have to for strong recommendations. You have to do your research.