SHOULD'VE, COULD'VE, WOULD'VE...When do I use these contractions?

First of all, here are the contractions:

should've=should have
would've=would have
could've=could have

Note: We use a past participle after each of these contractions. See below for examples. 

We use these words to express the impossible in the past tense. In other words, we use these contractions to look back on the past to think about what we would change if we could do it again.

Let's look at examples of each.

My car got broken into the other day!  I should've locked it...
You're in a traffic jam right now!  You should've taken the other freeway...

I went swimming all day yesterday and got a sunburn. If I'd had sunscreen, I would've used it!
They got lost on their way home from the beach.  They would've used a GPS if there had been Internet.

We went to New York last weekend.  We could've gone to the Statue of Liberty, but we didn't have enough time.
She didn't study for her final exam.  She could've gotten an 'A'...

Woulda Shoulda Coulda...You can say it but don't write it!

Now you know a bit more about the grammar and usage of these useful contractions!  Now go use them!