Help

So, i've just began learning Phyton in school but i dont understand a single thing. Currently im trying to a small program about it asking how my days been and it should be either a "yes" or "no" answert to it. After saying either it should also say a small message but im currently stuck trying to make the "if" command. It says its not "defined", can anybody help by what it means? How i should defy it more?