El Sustantivo

Nouns ending in 'o' are masculine: el medico, el dolor, imperio romano, pueblo, mensajero,

Nouns ending in 'a' are femenine: la enfermedad, la cama, la lengua, la ventana, la boca

There are some nouns which do not follow under either of these rules. Nouns denoting male beings are always masculine: un rey, un hombre
Nouns denoting female beings are always femenine: una mujer, una reina.

Nouns of Greek origin ending in 'MA' are generally masculine: el clima, el sistema
Days of the week are also masculine: el lunes, el martes, el miercoles

Nouns ending in 'ion' and in 'd' are generally feminine: la ediicion. la conversacion, la leccion, la ciudad, la bondad, la curiosidad