When to use the verb "Ser" and when to use "Estar"

[laughingwithfee]

My teacher gave us a formula that really helps:

When using estar, use x versus x, meaning you're comparing (insert whatever it is you're talking about) to itself.

When using ser, use x versus y, meaning you're comparing (insert whatever it is you're talking about) to a set of other things.

Example: Let's say we're talking about Paco. If I want to say that Paco is happy today, I would use estar, because Paco is happy compared to his usual emotional state. I would say, "Paco está feliz."

If I want to say that Paco is a happy person, I would use ser, because Paco is a happy person compared to everyone else. I would say, "Paco es feliz."

I hope this helped.