Chapter 2: Articles and formation of plurals

html5 video converter by v3.9.1

  2.7 Adjectives are used in their gender and form always according to the noun used

rojo = red

alegre = happy, glad
gordo = stout, big
la flor = flower
negro = black

  following examples show the way the accordance of adjective and noun work

la flor roja = the red flower

las flores rojas = the red flowers
el hombre gordo = the big man
los hombres gordos = the big men
la muchacha alegre = the happy girl
las muchachas alegres = the happy girls
el perro negro = the black dog
los perros negros = the black dogs

You can see the difference between the English and Spanish very clear in those examples. English does not change the gender (there is none actually) neither of the definite article nor of the adjective, nor does the number of the adjective change. Therefore, for the English the part of article and adjective stays unchanged.
Now Spanish brings the challenge to remember that articles (definite and indefinite) as well as adjectives need to be changed according to the gender and the number of the nouns.