Past Modal Verbs Include:

• Could have + past participle: something was possible in the past or one had the ability to do something in the past but didn’t.
• Couldn’t have + past participle: something wasn’t possible in the past, even if one had wanted to do it.
• Could have + past participle and Might have + past participle: used when we want to make a guess about something that happened in the past. It is unknown if what is being said is true or not true. This is expressing an opinion of what maybe happened.
• Should have + past participle:
  • 1) To express something that would have been a good idea but wasn’t done.When talking about yourself, you’re regretting what you did, and when talking to someone else, you’re giving advice.
  • 2) To talk about something that, if everything is normal and okay, we think has already happened. It is uncertain that everything is fine.
• Would have + past participle:
  • 1) Used as part of the third conditional (formed by using the past perfect after “if” with “would have” and the past participle used in the second part of the sentence—if + past perfect verb, … would have + past participle). This modal talks about the past and is used to describe a situation that didn’t happen and to imagine the result of the situation. Example: If he hadn’t eaten so much cheesecake, he wouldn’t have felt sick.
  • 2) Since “would” (and “will”) can also be used to show if you want to do something or not, “would have + past participle” can also be used to talk about something you wanted to do but didn’t. The if clause isn’t needed. Example: I would have gone to the party, but I was busy.

Special Note on May and Might
May and might have the same overall meaning and can normally be interchanged without a significant difference in meaning; however, when expressing possibility, might implies a smaller chance of something happening than may. May is viewed as indicating something is probable or more likely to occur. Might is viewed as suggesting something is unlikely or less likely to occur.

While may and might can often be interchanged, the exception is with negatives. May should not be used in a negative hypothetical as may is also used to denote permission. A reader may interpret the sentence to mean someone doesn’t have permission, or isn’t allowed, to do something.

Example: I may not go camping this weekend = I’m not allowed to go as opposed to I’m unsure if I will go.

In sentences similar to the example above, might should always be used.