If you have a green card, do you also need a passport to fly from?
Yes. You normally need a passport to enter a sovereign state (or economic group of states like Europe) - With one exception. The exception is, you dont need a passport to enter your country of citizenship - international law prohibits countries turning away their citizens (to prevent stateless people). Though if youre a citizen of more than one country, a country can revoke your joint citizenship leaving you just one place to go to. Travel between the USA and Canada used to be possible without a passport - but not any more.