Si la puerta no está abierta, está cerrada.
-if the door is not open, it is closed - cerrada is the opposite of cerrada
Marco está nervioso porque tiene un examen hoy.
Nervioso means nervious - he is nervious before his exam
Juan necesita dormir porque está cansado.
-he needs to sleep because he is tired (cansado)
Felipe está aburrido porque no encuentra nada interesante que hacer.
what do you do when you have nothing interesting to do? you are bored (aburrido, like Felipe)
Ana está enojada porque no encuentra el libro que busca.
Ana is angry - enojada - because she can't find a book she is looking for
Debes limpiar el piso porque está sucio.
here the apartment is dirty - sucio - and it needs cleaning