Esperanto: A Complete and Comprehensive Grammar/Adjectives

Adjectives
In Esperanto, all singular adjectives end in "-a," regardless of gender.

Adjectives agree in number and case with the nouns they modify; thus, all plural adjectives end in "-aj" and accusative plural adjectives end in "-ajn."

Ordinal numerals
Ordinal numbers are the adjective form of a number, and as so, adding the "-a" suffix is sufficient to change the word.

For example: