When you are speaking of a property something has, like for example, colour, you use "ser". I.e: "El edificio es blanco" If it is a property of an object that changes easily or changes all the time, like the STATE of traffic lights, then you use "Estar". I.e. "El semaforo esta en rojo" Like in Andy's example, "Sometimes I am a little bit selfish" , 'I AM' translates directly to 'Yo soy' and that's why we say "A veces soy un poco egoista". When it comes to people it does become a bit irregular, speaking about emotions and properties that describe us, we use "ser" and "estar" the same as with objects: "Estoy feliz", "Estoy emocionado", "Soy alto", "Soy delgado". But when speaking about behavior or situation (like being rich) we always use "Ser": "Soy rico", "Soy bueno", "Soy egoista". Hope my explanation is clear enough and will help you with that tricky bit of Spanish. If not, I'll review it when I get on my PC and format it properly too! Feel free to PM me with your questions, I'll try to answer ASAP :3 Buen día!