Is- when the subject is singular
are- when the subject is plural
was- past tense of is
were- past tense of are
sometimes you just gonna change like almost the whole sentence but you have to keep it grammatically correct :) btw, i hope my answers helped :)
Thank you samlrn
for simple present tense passive voice: "is and are" each followed by past participle form of the verb 
for simple past tense passive voice: "are and were" followed by past participle of the verb.
Example of simple present passive voice:  A ball "is dropped" from the top of the building.  Its active voice:  The strong wind "drops" the ball from the top of the building.

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We use 'is' and 'was' for singular subjects. Ex. Annie is good. Annie was good. However, 'was' should be used when referring to the past and 'is' should be used when referring to the present. 'Are' and 'were' should be used for plural subjects. 'Are' is used in the present time while 'were' is used when referring to the past. There are special cases though. One special case I could think of is using the pronoun 'you'. Ex. You is charming. *The subject might be singular, but this is a special case and should follow a plural verb so the right way is: You are charming. Hope this helps(: