French language Q&A Forum

I know that typically, retourner is used to mean "to go back" and rendre is used to mean "to give back." But on this page: https://www.lawlessfrench.com/grammar/auxiliary-verbs-2/ , which discusses using variable auxiliary verbs in the passé composé, it mentions that retourner can also be used transitively and in that case, it changes its meaning to "to give back." So in the passé composé, can retourner be used in the same way that rendre is?

For example, would both of these be correct?

1. J'ai rendu le livre à la bibliothèque.

2. J'ai retourné le livre à la bibliothèque.

Mettez au temps convenable

en 2021, nous (étudier) souvent en ligne

is avons étudié correct or étudiions is correct?

Which is more Correct and will I get mark for passe compose in a subjective exam.

Are both technically correct?

Est-ce plus correct à dire "Elle s'est rasé les jambes, elle les a lavées aussi" ou "Elle s'est rasé les jambes, elle les est lavées aussi"?

I thought we need to agree past participle with the number of people... I did, We brushed our hair = Nous nous sommes brossés les cheveux. However, kwizBot said, it was only nearly good, the right answer was: Nous nous sommes brossé les cheveux.
I'm confused, what's my mistake then? Did I misunderstand something?

I got the sentence “Nous n’avons pas eu....” wrong because I translated it as “We never had...”