1. In Spanish all the things and objects as well as abstract nouns have a gender. They are masculine or femenine. For example, lets say the noun ‘mesa’ (table) What do you think? Is ‘table’ a femenine or a masculine noun? In Spanish the noun mesa is a femenine noun. Why? No reason. It just is. It is not because a table looks femenine  It is arbitrary.

2. So, how do we know when a noun is masculine or femenine
So, how do we know when a noun is masculine or femenine? → We need to look at how the noun ends.

3. -o → juego, grupo, cuaderno -r → amor, sur
MASCULINE NOUNS generally end with these letters:
-O, -R;
-o → juego, grupo, cuaderno
-r → amor, sur
Some exceptions:
mano, radio ↓
They are femenine nouns even if they end in –o

4. -d → ciudad, nacionalidad -ón → canción, televisión
FEMENINE NOUNS generally end with with these letters:  
-A, -D, -ÓN;
-a → mesa, silla, puerta
-d → ciudad, nacionalidad
-ón → canción, televisión
Some exceptions:
agua, problema ↓
They are masculine nouns even if they end in –a

5. Nouns ending in –E can be both masculine or femenine.
-e → café (masculine)
calle (femenine)

6. So, how do you know when a noun ending in –e is masculine or femenine?
JUST LOOK IT UP IN A DICTIONARY!
Same for nouns that end in other consonants or vowels different from the ones above. (-z, l, i, u…)
→ You need to check in a dictionary.

7. Why is gender so important in Spanish
Why is gender so important in Spanish? Well, if a noun is masculine, then its descriptive adjectives and
articles will also need to be masculine. ↓
In order to describe something accurately, an adjective must match a noun in both gender and number.
For example:
La hoja es blanca. (The sheet is white.)
El  libro es blanco. (The book is white.)

8. Generally nouns have only one gender (they are only masculine or femenine, they cannot be changed)
ej. ‘mesa’ is always femenine (you cannot say ‘meso’)
However nouns that refer to people and animals can be changed.
For example:
gato / gata (male cat / female cat)
hermano /hermana (brother /sister)