what is the difference between must and ought to?
Must has similar meanings to should and ought, but is stronger or more definite. It expresses great confidence that something will happen, or that something is true; should and ought express less confidence. Compare:
The doctor said I must give up smoking. (an order which is likely to be obeyed)
You really ought to give up smoking. (a piece of advice which may or may not be followed)
Bob must be home by now. (=I'm sure he is home.)
Bob should be home by now. (=I think he is probably home.)
Should can be used instead of must to make orders and instructions sound more polite.
This form should be filled in in ink.
Applications should be sent by 31 January.
Should and ought can be used for predictions. Must is not used in this way.
It should be fine tomorrow. (BUT NOT It must be fine tomorrow.)
Should and ought can be used with perfect infinitives to talk about unfulfilled obligation in the past. Must is not used like this.
You should have been nicer to Annie.(BUT NOT You must have been nicer to Annie.)