Modal Verbs (II)

Expressing Possibility and Deduction

Possibility is conveyed by using some expressions like perhaps, maybe, possibly, probably, unlikely, improbably, surely and certainly, but also by means of modal verbs such as must, may, could, might and can't.

Possibility can be divided into three categories: certainty, possibility, impossibility. The use of these modals will be ranged according to these three parts, namely must for certainty (100%), may for possibility (50%), could or might for probability (25%) and can't for impossibility (0%).

Look at these examples:
a) Peter must be at the office (= Surely he is at the office).
     Pedro debe de estar en la oficina.
b) Peter may be at the office (= Probably he is at the office).
     Puede que Pedro esté en la oficina.
c) Peter might be at the office (= Unlikely, he is at the office).
     Pudiera ser que Pedro estuviera en la oficina.
d) Peter can't be at the office (= Surely, he is not at the office).
     Pedro no puede estar en la oficina.

In sentence (a) the chance that Peter is at the office is very big, maximum indeed 100%, we are completely sure that this is so. In (b) and (c) there is a possibility that Peter is not at the office, higher in (b) with a 50% and more remote in (c) with a 25% of certainty. And in (d), finally, we are sure that Peter is not at the office (0%), so it is impossible.

Logical deduction can be either positive or negative. Positive deduction is expressed with must and negative deduction with can't.  Observe these two situations:

Everyday my father gets home at 5 p.m.
Now it's five o'clock, and someone is knocking at the door. It must be my father.
Now it's two o'clock, and someone is knocking at the door. It can't be my father.

All this refers to present and future situations. For past situations perfect modals are used, but we will leave them for later, or probably next year.

Practice:
Exercise1, Exercise2, Exercise3, Exercise4.