While the same form of the definite article "the" is used for all cases in English, in Portuguese, the definite article must always agree with both the gender and the number of the noun it accompanies.

o = masculine singular (e.g.: o filho = the son / o carro = the car)
os = masculine plural (e.g.: os filhos = the sons / os carros = the cars)

a = feminine singular (e.g.: a filha = the daughter / a casa = the house)
as = feminine plural (e.g.: as filhas = the daughters / as casas = the houses)