Modal Verbs of Probability

Modal verbs are used  when we want to make a guess about someting. We choose the verb depending on how sure we are.

• Use 'must' plus the verb when you are 100% (or almost 100%) sure that something is the case.
• Use 'might' or 'may' to express an opinion that you think has a good possibility of being true.
• Use 'could' to express a possibilty which is one of many. This form is not as strong as 'might' or 'may'. It is just one of a number of possibilities.
• Use 'can't' to express an opinion that you are 100% sure is NOT true.

In the present: Subject Modal Base Form Objects I You He, She We They must be might be could be can't be at work.

In the past: Subject Modal Perfect Form Objects I You He, She We They must have left might have left could have left can't have left early.

Source http://esl.about.com/library/grammar/blmodalprob.htm