These are short forms used in spoken English: coulda - could have, shoulda - should have, woulda - would have. They are used when we talk about hypothetical situations. eg. I could have been a doctor. (but I wasn't) I should have listened to my mom. (but I didn't) I would have studied hard. (but I didn't) So, when we talk about being disappointed in how a situation is now, or how things happened, but in the past, there was a possibility for them to be different, we can use 'coulda, shoulda, woulda'. eg. A: I left my boyfriend. We fought so much, but now I regret it. Also, he could have been nicer... B: That's just coulda, shoulda, woulda. Don't think about that. Now it's over and you should move on. ______________________________ Hindsight is thinking about something that happened in past, but with more reason/experience. eg. I was so happy to work for $8 per hour on my first job. In hindsight, I realize that's below the minimum wage and they used my ignorance. 20/20 is the expression we use for someone who scores perfectly on the vision test. eg. The doctor said my sight is 20/20, so I can become a pilot. Hindsight is 20/20 means that looking back in the past, you can perfectly see what was going on (but it's difficult to see the present/future) eg. Oh, I was so naive when I was young. I believed all those people were my friends. However, hindsight is 20/20!

"coulda, shoulda, woulda" is short for "could've, should've,would've" and can be used in a couple of ways. First, I could say that if someone is criticising me too much on something I did. For instance, I failed an exam and my friend keeps telling me in an irritating way that if I had not partied all the time, I would have passed the exam. I could respond and say "coulda, shoulda, woulda". That's a little sarcastic way of telling my friend it is all too late now and complaining on what I should've done doesn't help me. Secondly, this expression can be used as a lightly critical response to someone continually making excuses for what they should have done. I keep complaining to my friend I would have got great marks for my exam if I had made time to study. My friend could simply reply "Coulda, shoulda, woulda". My friend is telling me it is too late now and stop complaining. Hindsight is 20/20 is generally a similar expression but not so colloquial. It's generally used in the same manner as "coulda, shoulda, woulda" but the tone is softer and it is not quite as mocking. It's more polite way of giving the same message. Someone is telling me they knew I was going to fail my exam because they saw I wasn't studying. If I say "hindsight is 20/20", I am telling them I know you are right but it wasn't so obvious to me then. Easy to tell me now about what I should have done then when you know everything that has happened.