Nueve vs. nuevo
Although the Spanish words nueve and nuevo look and sound similar, they have completely different and unrelated meanings. One is a number, and the other is a descriptive adjective.
Nueve
A1The word nueve is a cardinal number that means nine (9). It is invariable, meaning it does not change its form regardless of the noun it accompanies.
Mi hermana tiene nueve años.
(My sister is nine years old.)
Son las nueve de la noche.
(It is nine o'clock at night.)
Compramos nueve boletos para el concierto.
(We bought nine tickets for the concert.)
La reunión es el nueve de mayo.
(The meeting is on the ninth of May.)
El nueve es mi número favorito.
(Nine is my favorite number.)
Nuevo
A1The word nuevo is an adjective that means new. As an adjective, its form must agree in gender and number with the noun it modifies. It changes to nueva for feminine nouns, nuevos for masculine plural nouns, and nuevas for feminine plural nouns.
Tengo un teléfono nuevo.
(I have a new phone.)
Ella vive en una casa nueva.
(She lives in a new house.)
Necesito comprar unos zapatos nuevos.
(I need to buy some new shoes.)
Tenemos muchas ideas nuevas para el negocio.
(We have many new ideas for the business.)
El profesor es nuevo en la escuela.
(The teacher is new to the school.)
Summary
In short, nueve is the number 9 and always stays the same. In contrast, nuevo is an adjective meaning new and must change its ending to nuevo, nueva, nuevos, or nuevas to match the gender and number of the noun it describes.