Regular adjectives in Spanish are those that follow this rule to form their singular:
ending -o for masculine
ending -a for feminine
Have a look and listen to these examples:ending -a for feminine
Notice how the endings are different on each phrase, as the first one is refering to a man (hombre) and the second one is refering to a woman (mujer).
Here are more examples:
El gato pequeño es bonito.
The little cat is cute.
Tengo una amiga simpática.
I have a friendly friend.
See also Forming regular plural masculine and feminine adjectives
