Need Latin help: subjects in sentence with a passive voice

[cintinative]

We are confused over here.

We have learned that the "subject" does the action, however a recent Latin lesson has confused us.

For example, in the sentence,  "The youth is chosen to be a leader of the soldiers"  

"is chosen" is passive, so the "youth" is not doing the action. In English, I would not mark "youth" as the subject.  However, there is no person listed doing the action in this sentence.  Perhaps we could modify it to be "The youth is chosen by the president to be a leader of the soldiers." In that case, my understanding is that "president" would be the subject. He is the one doing the action. 

Here is the confusion. In our Latin curriculum, they are saying to label "youth" as Nominative, which is the case for subjects and predicate nominatives.  In our opinion, "youth" is neither of those. 

They also said that "youth" cannot be a direct object with the passive voice in Latin, but did not explain why.

Can anyone help clarify this for us? My son is very frustrated. I did watch the video but it did not clarify these things. 

Here's another example of theirs:  The city is destroyed by a great fire. 

For this sentence, they translated "city" using the Nominative case and "fire" using the "ablative" case.   This sort of makes sense because of the "by" preposition indicating Ablative, but technically the "fire" is doing the action here.  

 

Help?? TIA!!!

 

ETA: 

It looks like we are not remembering this rule.

In a sentence using passive voice, the subject is acted upon; he or she receives the action expressed by the verb. The agent performing the action may appear in a "by the..." phrase or may be omitted.

 

Edited September 25, 2019 by cintinative

[forty-two]

"The subject does the action" guideline only applies with active voice.  With passive voice, the subject is *not* the actor - in fact, this website says that what makes active voice active is that the subject is doing the acting.  Likewise, what makes passive voice passive is the very fact that the subject is being *acted upon* by the verb - the subject is *passive*, not active.  So in "The youth is chosen to be a leader of the soldiers", youth *is* the subject even though youth isn't the actor - being in passive voice turns things around, makes the subject the one acted upon by the verb, not the doer of the verb.

The whole how to reconcile "subject does the verb" rule with passive voice really confused me, too.  Hope this helps.

 

[cintinative]

It looks like we are not remembering this rule.

In a sentence using passive voice, the subject is acted upon; he or she receives the action expressed by the verb. The agent performing the action may appear in a "by the..." phrase or may be omitted.

My son is still confused, but I think I get it.  😃