Past Models

past modal verbs are used to talk about things that didn't really happen in the past.

Could have + p.p.

1. Could have + past participle means that something was possible in the past, or you had the ability to do something in the past, but that you didn't do it.

Example
 I could have finished my homework but I was lazy.

2. Couldn't have + past participle means that something wasn't possible in the past, even if you had wanted to do it. 

Example
 I couldn't have gotten the concert ticket of Queen but I really wanted to go.

Should have + p.p.

1.Should have + past participle can mean something that would have been a good idea, but that you didn't do it. It's like giving advice about the past when you say it to someone else, or regretting what you did or didn't do when you're talking about yourself.

Example
  I should have gone to his birthday party. I regret that.

2.Shouldn't have + past participle means that something wasn't a good idea, but you did it anyway. 

Example
 I shouldn't have eaten much snacks because I'm on a diet.

Would have + p.p.

1.Would have + past participle is the part of third conditional.

Example
 If I had had enough money, I would have bought a new house.

2.we can also use would have + past participle to talk about something you wanted to do but didn't. This is very similar to the third conditional, but we don't need an 'if clause'. 

Example
 I would have invited you to the party, but I didn't know your number.