Use of have to and must

Difference between have to and must.

Have to and Must are used to talk about necessity.

Example: In order to defend our country we have to/must build up a strong army.

Obligation:

Must is used mostly to talk about the feelings and wishes of the speaker and listener. Have to/have got to is used mostly to talk about obligations that come from ‘outside’ – for example from laws, regulations, agreements and other people’s orders.

Compare:

I must walk in the morning. (I want to do it as an exercise.) (This is my own decision)

I have to walk in the morning. (Doctor suggested me as a treatment.) (Someone else is forcing me to do it)

Will have to can be used to talk about future obligation, on the other hand, have to/have got to is preferred when arrangements for the future already have been done.

you will have to find a job When you leave school. (Future obligation)

I have got to go for a job interview tomorrow. (It is already arranged by calling you before.)