velma

I always thought they were interchangeable, but usted was used in more formal settings. You are going to use usted instead of tu if speaking to someone older than you to show respect. That's what I was taught.

It is, but usted can also be used when talking to someone you don't know personally like a teacher anybody that is not a friend of yours. While tu is used when talking to someone you know personally like a friend, sister, or brother.

The best way I learned the little Spanish that I do know is by learning a new word everyday. Using it in a sentence and downloading a English to Spanish translator to practice saying it and hearing it. I would sometimes watch movies but that did not help me at all. I also increased my vocabulary by writing down Spanish words from a dictionary and reading Spanish books and recognizing words that were similar to English.