"There are girls."
Translation:Il y a des filles.
Oh god these DE + DEFINITE vs DEFINITE vs INDEFINITE articles are proving to be troublesome.
IF 'Hello girls!' is translated to 'Bonjour les filles' AND NEVER 'Bonjour des filles' then it only makes sense that 'There are girls' would be 'Il y a les filles' and not 'Il y a des filles' .
"bonjour les filles" is an adress to all the girls present. In that case, the French use the definite article, because these girls are considered as defined.
"there are girls" means that there is a certain number of girls, which is not definite, so the French use the indefinite article. Note that the singular if this sentence is "there is a girl" which is indefinite. The trouble here is that in English, there is no plural to "a/an" while in French, there is a plural form to un/une, which is "des".