Use ‘have to’ to express responsibility or necessity that exists in the present. Use ‘will have to’ to express future responsibilities. Use ‘had to’ to talk about obligations that existed in the past.
Have to is used as a regular verb. That means questions and negative forms are made with do. Note that have to is mainly used to talk about obligations that come from outside sources.
- I have to get up early tomorrow. (Affirmative)
- I do not have to get up early tomorrow. (Negative) (NOT I have not to get up early tomorrow.)
- Do you have to get up early tomorrow? (Interrogative) (NOT Have you to get up early tomorrow?)
- She had to leave early yesterday.
- She did not have to leave early yesterday.
- Did she have to leave early yesterday?
- She will have to wait for an hour.
- She will not have to wait for an hour.
- Will she have to wait for an hour?
- He has to go.
- He does not have to go.
- Does he have to go?
In British English, must is also used to talk about necessity. It is mostly used to talk about the feelings and wishes of the speaker and the hearer.
- I must find a good job. (Because I want to.)
- I have to find a good job. (Because Dad has asked me to find a good job.)
- I must stop smoking. (I want to.)
- I have to stop smoking. (My doctor has asked me to stop smoking.)
Must is only used to talk about present and future obligations.
The negative form of ‘have to’ is used to express the idea that something is possible but not necessary.
- She does not have to wait for me. (It is not necessary for her to wait for me.)
The negative form must not is used to express the idea that something is prohibited.
- You must not play with fire. (It is prohibited.)
- She must not do that.