The main modal verbs that express probability are described here in order of certainty. Will is the most certain, and might/could are the least certain.
Will.
Will and won’t are used to predict a future action. The truth or certainty of what is asserted is more or less taken for granted.
e.g. His latest book will be out next month.
b) Will and won’t are also used to express what we believe or guess to be true about the present. They indicate an assumption based on our knowledge of people and things, their routines, character and qualities.
e.g. Leave the meat in the oven. It won’t be cooked yet.
It’s Monday morning, so I guess right now Sarah will be taking the children to school.
Must and can’t.
Must is used to assert what we infer or conclude to be the most logical or rational interpretation of the situation. We do not have all the facts, so it is less certain than will. Must in this meaning is not used to speak about the future.
e.g. You say he walked across the Sahara Desert! He must be mad!
You must be joking! I simply don’t believe you!
