"She has red pants."
Translation:Lei ha i pantaloni rossi.
"Lei ha i pantaloni rossi" would be "she has THE red pants", showing that they are talking about a specific aforementioned pair of pants. Not including the article shows that this girl has red pants, but they're not talking about a specific pair of red pants. In english the difference is apparent as well.
She has the red pants -indicates specific pants
She has red pants -much more general hope it helps.
To me, adding the article in Italian, but not English, only makes sense if we are talking about the trousers she is currently wearing. Because you also translate "she has long hair" as "ha i capelli lunghi". So I assume in such cases, Italians prefer to use the specific form even if English doesn't.
rossa (red) will agree with the noun it modifies (singular OR plural, masculine OR feminine) like most adjectives
rosa pink is an exception, and is invariable (used without variation with singular AND plural, masculine AND feminine)
le rosse mele the red apples le rosa mele the pink apples
i rossi elefanti the red elephants i rosa elefanti the pink elephants
In both English and Italian, the definite article is used when the context is that a definite pair of red pants is being discussed. Just as with English, the speaker decides when he is speaking of any pair of red slacks or a specific pair. The Italian sentence in this exercise is obviously speaking of a specific pair. Conversely, the English sentence is indefinite to the extent that it doesn't specify whether she has more than one pair of pants. Depending on context, this English sentence can be taken as either definite or indefinite.