SUMMARY OF UNIT 6

EXAMPLES Lincoln should have had better protection. You should have voted in the last election. Every vote is important. In 1962, a nuclear attack was avoided. It may have been because of good diplomacy, or might have been because the Soviets feared a world war.

When John Booth entered the theater, people might have thought he was an actor in the play that night. People thought Nixon must have been nervous because he was sweating. The whole world must have been afraid in October 1962. In 1962, a world war could have started, but it didn’t.

You could have watched the movie Lincoln, but you weren’t interested. A: I voted in the last presidential election. B: You couldn’t have voted. You weren’t even 18 at that time. When I was younger, I could name all the presidents in my country, but now I’ve forgotten. I was able to read Lincoln’s speech without using a dictionary.

McCain didn’t use a computer. He had to depend on his wife. Booth must have been planning the assassination for some time.

EXPLANATION We use should have + the past participle to comment on mistakes or regrets or to rethink the advisability of a past decision.

May/might/could have + the past participle shows possibility about a past action or event.

Must have + the past participle shows a logical conclusion or deduction about the past.

Could have + the past participle shows a past direction not taken, a past possibility that didn’t happen, or a past suggestion that wasn’t followed.

Couldn’t have + the past participle can show disbelief or an attempt to disprove a previous statement.

To express past ability, we use could + the base form or was/were able to + the base form. In affirmative statements, could means used to be able to. To show success in doing a single action, we use was/were able to for affirmative statements. Had to + the base form shows an obligation or necessity in the past. For a continuous meaning of a past modal, we use modal + have been + the present participle.