Formal vs. Informal
Can someone explain to me the difference between formal and informal when speaking in spanish? or writing in spanish.
In formal speaking or writing you'd use the "usted" form to conjugate your verbs. For example, "¿Cómo está usted?" In informal speech and writing you'd use the "tu" form. For example, "¿Cómo estás?"
Formal should be used with teachers, strangers, elders, etc.
Informal should be used with friends, children, etc.
You would always say "usted" when talking to an important person. No, not important as in a superstar (I may be wrong on that :D) but like an elder or your teacher or your boss whoever it may be. If the are older you us usted or if the are someone you listen to and follow, you use usted.
And to further expand, when you meet a stranger at a party, etc - and the two of you are of the same generation - you may use Tú form right away.
Since criminals are subhumans and have no respect for tradition or the dignity of their victims, they will always use the Tú form - even if the victim is elderly. In this situation, the victim will still respond in Usted form to create distance.
In any situation, Usted form is handy for establishing distance.
There is even a verb in Spanish that you use specifically to request that you use the "tú" form with someone. If only I could remember what it is...
Marianne is absolutely correct. I will merely put it in a "nutshell" for you: if you address the person by his first name (or an equivalent, such as "mom"), use the informal. If you are not on a first name basis with the person, use formal.