Modal Verbs are very useful words. The most common are: will, can, could, would, should, must, might, and may. The rules for using them are easy:
♦ The negative is ‘not’ or ‘never’: You should not be late for work. I will never leave you!
♦ Modals are always followed by a Base Infinitive: be, go, have, live etc: Could you help me? I might play football this weekend. Would you be quiet please?
If I asked you the meaning of ‘must’, you would probably talk about obligation. Yet, this is not how we usually use this word. In fact, we more often use the Modal Verb ‘must’ (as well as other Modal Verbs) for Deduction or Speculation.
I have found a fabulous English blog called My English Blog which is written by aliciateacher2.wordpress.com To read her excellent post on Modal Verbs of Deduction or Speculation, click on the link below.
Source: Modal Verbs of Deduction or Speculation