[Grammar] past participle in passive sentences

[egerol1]

Do we have to use the third verb form(past participle) of a verb in passive sentence like this:

-as shown here

-this is a song written by whose name is unknown

-it's a broken leg

or just let me ask it in another way
Why do we use past participles here and not simple past form?

Thanks for your help

 

[bhaisahab]

Do we have to use the third verb form(past participle) of a verb in passive sentence like this:

-as shown here

-this is a song written by whose name is unknown

-it's a broken leg

or just let me ask it in another way
Why do we use past participles here and not simple past form?

Thanks for your help

These are not sentences, neither are they passive. The second one makes no sense at all.