Spanish nouns can be either masculine or feminine. Masculine are the nouns that end in –o:
el libro (book)
with the exception of such feminine nouns as la mano (the hand), la radio (the radio), la foto (the photo), la moto (the motorbike).
International words that end in –ma and have the same meaning as in English, and words that end in–dor and –aje also are masculine:
el tema (theme), el problema, el programa
el trabajador (worker)
el equipaje (luggage)
Feminine are the nouns that end in –a, –ción, –sión , –is, –z and –d:
la casa (house)
la canción (song), la tensión
la edad (age), la red (net), la facultad (faculty)
la voz (voice), la vez (time/occasion)
la crisis (crisis), la tesis (thesis)
Exceptions: masculine nouns el día (day), el planeta, el mapa, el análisis and about 20 very unfrequent words more.
All the other noun endings do not indicate the gender of a noun:
el árbol (tree) – la cárcel (prison)
and in such case the gender can be identified through checking the corresponding dictionary marks (m – masculino, masculine, or f – femenino, feminine) or by the gender of the noun determiners and attribute words.
The gender of such nouns is easier to memorize together with the article from the very beginning.
Many proper names and nouns derived from adjectives take the gender of the words that determine them, irrespective of their ending:
Days of the week: el (día) lunes, el martes, el miércoles…
Colours: el (color) amarillo, el rosa, el violeta,…
Equally, geographical names – rivers, lakes, seas, oceans, waterfalls, mountain peaks and deserts – are masculine:
el Sahara – the Sahara desert
el Ebro – the Ebro river
el Aconcagua – the Aconcagua peak