Study them!

Ill give a quick example before you study. A main reason for passive voice is to not draw attention to the true subject. "John broke the window. It was not me." This person wants it to be clear whose felt it is. John is the subject. This is called active voice. This is the natural way a sentence is made. One person or thing (called the subject) DOES something to another. This second thing is called the object. "The winnow was broken." This is very passive. The speaker is not ready to cast blame. In a passive sentence The object is moved forward to become the subject. "The window was broken by John and I." All the information is given in passive voice which places the window in the role of the subject. The person doing the action is moved to the end, taking on the placement of what would have been the object.

We use the passive for two big reasons; we don't know who or we don't care who. We form the passive with some form of "be" and the passive participle. Spanish is spoken in Mexico. http://www.google.com/#hl=en&gs_nf=1&cp=11&gs_id=16&xhr=t&q=grammar+passive+voice&pf=p&sclient=psy-ab&pbx=1&oq=grammar+pas&aq=0&aqi=g4&aql=&gs_sm=&gs_upl=&bav=on.2,or.r_gc.r_pw.r_qf.,cf.osb&fp=5f09a43e32662866&biw=1280&bih=918 There are lots of references on the Internet!