Formation

Verb (“a”colum verb or “nai” negative stem) ＋　れる・られる

(Class I)
　～れる:  Plain Negative form "a" column without the negative word "nai" + れる
　言う/言わない　→　言われる (be told)   v
　聞く/聞かない　→　聞かれる (be heard)

(Class II) 
　～られる:  Change る to られる
　食べる　→　食べ.られる (be eaten)
　建てる　→　建て.られる (be built)  このビルは二年前に建てられた。   This building was built two years ago.

Note: The passive of group 2 verbs happens to be the same as the potential and honorific. Which can only be understood/interpreted by the context:

(Class III    irregular verbs)
　来る　→　来(こ)られる
　する　→　される

Compare the passive and active examples below.

(Active) 
　子供がジュースを飲みました。
　The child drank the juice.
(Passive) 
　ジュースが子供に飲まれました。
　The Juice was drunk by the child.

Sample Passive sentences:
長い間待ってやっと私の名前が呼ばれた。 
After waiting for a long time, finally my name was called. 

言われなくても、分かってるよ.  
Without being told, I know that already!  (I know. Do not tell me.)

日本では日本語が話されています。  
Japanese is spoken in Japan.  

佐藤さんは田中さんにパーティーに招待されました。  
Sato was invited to the party by Tanaka.  

日本ではこの映画（えいが）はみんなに知られています。 
This movie is known to everyone in Japan.  

Note: Using passive form to show politeness or Annoyance.

 “Using passive form is another more polite way to express an action. In Japanese, a sentence is usually more polite when it is less direct. For example, it is more polite to refer to someone by his or her name and not by the direct pronoun "you". It is also more polite to ask a negative question than a positive one. (For example, 「しますか？」 vs. 「 しませんか？」) In a similar sense, using the passive form makes the sentence less direct because the subject does not directly perform the action. This makes it sound more polite. Here is the same sentence in increasing degrees of politeness.　　 This clause is taken from: http://www.guidetojapanese.org/learn/grammar/causepass　”

先生は納豆（なっとう）が食べられる  Potential: My teacher can eat natto.      The が indicate it as potential.

先生は納豆を食べられた could be many things:

Note: The passive of group 2 verbs happens to be the same as the potential and honorific, which can only be understood/interpreted by the context: 
Honorific: Teacher ate natto.
Potential: Teacher could eat natto.
Indirect passive: someone ate natto. (~and teacher was unhappy about it)

There is also an indirect passive, which doesn't exist in English. It implies the other person is annoyed by the action.  Perhaps it could be translated as "person A did something AT person B"

ビールは田中に飲まれた
Ｔhe beer was drunk by Tanaka。

先生は田中にビールを飲まれた
(Literal translation: The teacher was drunk beer at by Tanaka。)
The teacher was annoyed by Tanaka’s drinking the teacher’s beer.