In Spanish, all nouns are either masculine or feminine, whether they denote a person, a thing, a place or an idea.

Masculine

Masculine nouns usually end in –o: 

• el libro (book)
• el curso (course)
• el colegio (school)

Compound nouns (verb+noun) are always masculine: 

• el cortaúñas (nail clippers)
• el rascacielos (skyscraper)

Many nouns that end in –ma (those of Greek origin) are masculine: 

• el problema (problem)
• el tema (theme)
• el sistema (system)

Exceptions to these rules include: 

• la mano (hand)
• la radio (radio)
• la alarma (alarm)
• la pluma (feather)

Feminine

Feminine nouns usually end in –a: 

• la lengua (language)
• la casa (house)
• la escuela (school)

Nouns that end in –ción, –sión, or –ía are feminine: 

• la conversación (conversation)
• la televisión (television)
• la economía (economy)

So are the nouns ending in –dad, –tad, or –tud: 

• la universidad (university)
• la amistad (friendship)
• la actitud (attitude)

Also, those ending in –umbre or –za: 

• la costumbre (custom)
• la pobreza (poverty)

Exceptions to these rules include: 

• el día (day)
• el mapa (map)
• el sofá (couch)

Feminine nouns that begin with a stressed a or ha syllable use the masculine article in front of their singular forms, but the feminine article when in plural:

• In singular
  • el aula (classroom)
  • el área (area)
  • el hacha (axe)
  • el hada (fairy)

• In plural
  • las aulas (classrooms)
  • las áreas (areas)
  • las hachas (axes)
  • las hadas (fairies)

When referring to people, e.g. with an occupation, some masculine nouns end in a consonant: 

• el señor (sir)
• el profesor (male teacher)

and they have a corresponding feminine form that ends in –a: 

• la señora (madam)
• la profesora (female teacher)

Nouns ending in a consonant or the letter e can be either masculine or feminine. They have to be memorised. For example:

• el coche (car)
• el sol (sun) 
• el limón (lemon) 
• el pez (fish) 
• la calle (street) 
• la noche (night)

Some nouns have the same masculine and feminine forms. In these cases, the article indicates the gender: 

• el estudiante (male student) – la estudiante (female student)
• el artista (male artist) – la artista (female artist)

Finally, there are nouns that can be both masculine and feminine, but have a different meaning depending on gender:

• el frente (front) – la frente (forehead)
• el corte (cut) – la corte (court)
• el pendiente (earring) – la pendiente (slope)