Should, which means deberia works the same in English as in Spanish, but could and would don't.

Should is used in opinions:

I think you should take an English class if you plan on traveling to England.

Could has three meanings:

1) Conditional
2) Past of can
3) A possibility of: might, may

1) As a conditional, a sentence with could has an if in it. It is related to can. The verb is conjugated in the past form:

Example: I could go to Europe if I had the money.

2) As the past of can, a sentence with could looks like this:
When I was a child, I could climb that tree.

3) As showing a possibility of something, a sentence with could looks like this: It could happen.

* A sentence with could being used as might or may is generally meant as being defendant on something.

* Could can only be used in the past and present and not in the future. In the future you use will be able:

Would is similar to could, but is defendant on a decision rather that an ability to do something.

Example: I would go to the beach today if the weather was better.
Example: When I graduate I will be able to get a good job.
Would is used as a conditional also, so it goes with an if, but would is related to a choice, where as could is related to an ability to do something:
Example: I would go to Europe if my girlfriend came with me.

      Practice sentences

1) I _________________ have gotten to work on time if my car had started.

2) I __________________ have gone to work if it wasn't raining.

3) You didn't wash the dishes!? Why not? You ______________ have.

4) I think we ___________ go to the beach today.

5) I ____________ join the Navy if I knew how to swim.

6) I __________ if I ___________ but I can't so I won't

7) I _________ have bought you a hamburger if I knew you wanted one.