I wrote "Kiom da lingvoj parolas vi", and it marked it as wrong. It shouldn't matter where the 'vi' is in this case.
I came here to say this.
"Kiom da lingvoj" is a direct object phrase for the verb "parolas", so I used the -n suffix to signify this and entered "kiom da lingvojn vi parolas" as my answer. It was marked wrong.
"Da" doesn't accept accusative.
Thanks, I just realized the Da/De hints say this too. https://www.duolingo.com/skill/eo/Da_De
Mi parolas la anglan, la portugalan kaj hispanan.