This document outlines the uses of modal verbs like must, may, might, could, and can't to indicate different levels of certainty or possibility in deductions about situations. It explains that must is used when something is definitely true based on evidence, may/might are used when something is a 50% possibility, and can't/couldn't are used when something is definitely not true or impossible. It also provides examples for how to use these modal verbs to talk about expectations for the future or past.

2.  When you are sure something is TRUE: you
guess it from the context or evidence.
Ex.: He must be rich. He drives a Ferrari.
He hasn’t come to class. He must have
fallen asleep. He must be sleeping.

3.  When you are not sure something is true (50-
50):
Ex.: He hasn’t come to class. He
might/may/could be ill.
Ex.: He hasn’t passed the exam. He
may/might not have studied very much.
(Don’t use could in negatives with this
meaning)

4.  When you are sure something isn’t true or is
impossible:
Ex.:He can’t be Spanish. He only speaks
English.
He got a 0 in the exam. He can’t/couldn’t
have studied very much….
Mustn’t is NEVER used with this meaning,
only for prohibitions. The opposite of must is
always can’t in deductions.

5.  He may not have done the homework. He was
busy: Maybe he didn’t do it but it’s possible
he did it.
 He can’t have done the homework. He was
partying all weekend. It’s impossible he did it.

6.  For something we expect to happen:
Ex.: The classes aren’t complicated this year.
The exam should be quite easy.
The classes weren’t complicated. The exam
should have been easy.