You are invariably useful!
As we are in Beta, perhaps the ever-alert and ever-attentive Team might note Deponents as requiring some Tips guidance as above.
I don't believe we've broached the Passive yet, let alone had guidance to verbs Passive in form but Active in meaning.
For 're-treads', friends reunited; for newbies, quite mind-blowing?
It's a saying from Cato the Elder that means 'I also think that Carthage must be destroyed', and something of a 2nd Century BC mic-drop. The grammar needed to explain it is somewhat beyond this duolingo course; it is using something called the periphrastic passive construction (Carthago delenda est), but subordinated to a verb like censeo (I think), the verb becomes infinitive and the subject and predicative gerundive both become accusative.
In direct speech (oratio recta) we have a subject in the nominative and a finite verb (e.g. Carthago... est); in indirect speech (oratio obliqua, following verbs of thinking, saying, etc.), since the main sentence now has a nominative subject who is thinking or saying and a finite verb (e.g. Brutus dicit; Cato censet; censeo), the statement being thought or said is transformed so that its subject is in the accusative, and its verb is in the infinitive (e.g., Brutus dicit Carthaginem... esse). Anything predicated of that indirect subject will also be in the accusative (e.g. censeo Carthaginem delendam esse, instead of “Carthago delenda est”).
Different languages, different usages. In English, "talk" and "speak" are used in different ways. Just like in English we have one "to be" and one "to know" but in Spanish there is "ser" and "estar", which are not interchangeable, and there is "saber" and "conocer", which are not interchangeable.