Usted. It is the polite way to address and adult that you don't know, a teacher, a stranger, the elderly, etc
Usted = you
Example sentence: ¿Usted tiene un pluma que puedo usar? Do you have a pen I can borrow? (also side note: any word ending with -ma is masculine. Pluma is slang for pen and it is masculine. You can also say boligrafo for pen.)
