Modal verbs

Deduction and speculation

We use might/may/could (not) + infinitive to say that something now or in the future is possible.

• The soldier might be alive.
  (= It's possible that the soldier is alive.)

• He might not be dead. (NOT: He could not be dead.)
  (= It's possible that he is not dead.)
  
  
• He could live in New York.
  (= It's possible that he lives in New York.)

We use must, might/may/could or can't + infinitive to speculate about situations in the present.

• He must be alive.
  (= I am sure he is alive.)

• He might/may/could be alive.
  (= I think it is possible he is alive.)

• He can't be alive.
  (= I am sure he isn't alive.)

We can also use might/may/could + infinitive to say that a future situation or action is possible.

• I might go to New York in August.
  (= It's possible I'll go to New York in August.)

We can use must/may/could, might or can't + be + verb + -ing to speculate about activities in progress in the present and about temporary situations in the present.

• Mary might/may/could be having lunch.
  (= It's possible that Mary is having lunch.)

Use must, might/may/could or can't + have + past participle to speculate about the past.

• She must have been very unhappy.
  (= I am sure that she was unhappy.)

• She might/may/could have arrived late.
  (= It's possible that she arrived late.)

• He can't have heard the phone.
  (= I am sure that he didn't hear the phone.)