"The years pass and certain things change."

[hanan1948]

you say that "alcune cose" can replace "certe cose". does not "alcune" mean "some" and not "certain" ?

[f.formica]

"Certain" can also mean "some": to quote the Merriam-Webster dictionary, "of a specific but unspecified character, quantity, or degree <the house has a certain charm>". That's what duolingo's pointing at, in this case the unspecified quantity; "certo" and "certain" have similar contexts.

[oktaya]

Shoot. I read it as "three years" . And it was my last heart :D

[Bunny2013]

Why is is "certa gente" but "certe cose"? (Why is "people" singular, but "things" plural?)

[peter2108]

la gente is always singular. le persone is always plural. Both mean people. Perhaps is the ambiguity in the Italian idea of "people" - does it refer one thing or many things? We British seem to have a similar ambiguity about sheep.