why is it under "vous", lourd becomes lourds?
If "vous" is used as the "you" formal, 2nd person singular, then "You are heavy" translates into "Vous êtes lourd/lourde".
If "vous" is used as the "you" plural, 2nd person plural, then "You are heavy" translates into "Vous êtes lourds/lourdes".
How do we identify whether the sentence is referring to you (singular) / you (plural)?
From the verb conjugation; otherwise, from context. If context is insufficient, then it's ambiguous.
Isn't it both "êtes" in singular formal and plural? And the adjective conjugations sounds identical with this exercise as well. so conversationally it wouldn't have a difference, right?
just it can be plural,the answer is right
I answered 'Tu es lourd' and it was wrong. But how am I supposed to know that You Are is referring to Vous etes or Tu es?
I think all three answers from English are correct. There's no context to understand that Duo talks about one single person.
Does 'Lourdes' refer to a group of girls?
Yes it does.