Quiz question requiring writing passive voice

Question

The passive voice in several examples where we needed "was sent" and "had prepared" used plus que parfait ie) était envoyé and avait préparé, but for "the students were welcomed", my use of étaient accueillis was incorrect and the correct answer was the p.c.: Les étudiants ont été accueillis was correct. This seems illogical to me. Please explain the difference.

Accepted Answer

Hi Ann,
Without the full context it is difficult to fully explain why the P C is the correct answer but the meaning is very different as -
ont été accueillis = were welcomed
as opposed to -
étaient accueillis = were being welcomed
Just a note on the passive use of envoyer 
La lettre était envoyée par  ( passive voice - imparfait), 
La lettre a été envoyée par.... ( passive voice - P Composé)
Avait préparé which you quote is the active voice of the verb préparer ( it uses avoir)
Avait été préparé would be passive e.g.
Le repas avait été préparé à l’avance = The meal had been prepared in advance 
And is in the plus-que-parfait 
a été préparé = was prepared ( P C)
était préparé = was ( being) prepared ( Imparfait)
Hope this helps!

Answer

La lettre était envoyée. -- The letter was sent. This sentence is passive voice imperfect tense. The second phrase you mention, "had prepared", doesn't look like it's asking for passive voice to me, though. Can you post the entire sentence?étaient accueillis is passive voice imperfect, whereas ont été accuellis is passive voice passé composé.You might want to review the formation of passive voice: https://french.kwiziq.com/revision/grammar/how-to-form-the-passive-voice-with-compound-tenses-la-voix-passive

Answer

But why do we use the passé comp here? My problem is not understanding which tense is which. I can identify each tense, I just don't understand why were welcomed is translated using the passé composé.

Answer

Nothing deep here. The passé composé is used for the usual reason: a single event in the past.

Answer

Thanks. But I still don't understand. Was the letter being sent not a single event in the past? How can I differentiate when to use each tense?