When it's 4O'Clock and i'm Awake cuz there's Rain on a Spring Day and I Need U to Hold Me Tight because i'm Lost and there's Danger like Fire so Save Me, Don't Leave Me or Run away because you got da Best Of Me...
Am I Wrong? Everybody say N.O. Let Me Know, don't Lie.
~21st Century Girl/A.R.M.Y.
사람 means "person"; adding -들은 makes it plural, so 사람들은 means "people." 언제 means "when." 행복 means "happy"; adding -합니까? makes it a question, so 행복합니까? implies "are you happy?" All together, 사람들은 언제 행복합니까? roughly translates to "people -- when are they happy?" Have to get used to their sentence structure, kind of poetic actually.
사람들은 언제 행복합니까? implies "When are people happy?"
Though we may not use this specific sentence in our everyday conversation, it's versatile so you can still learn from it. For example, insert 너 (you) in place of 사람들은 (people) to get: 너는 언제 행복합니까? (When are you happy?). Or insert 왜 (why) in place of 언제 (when) to get: 너는 왜 행복합니까? (Why are you happy?) Or simply say: 행복합니까? (Are you happy?)
Hope this helps! :)