"We have sons."
"We have sons." What role or function is the word "sons" playing in this sentence? Are they the subject ( = doing the action, in this sentence, "having"), or are they the object ( = being affected by the action, i.e., "being had" ) ?
"We have THEM." In this sentence, "sons" are the object, because SONS are what we have: Fīliōs habēmus . fīliōs is the accusative plural form of this 2nd declension noun, fīlius, fīliī , m., son.
To use the form fīliī , the nominative plural form, we'd need a sentence in which the sons were doing something: Fīliī patrem amant , "The sons (THEY) love their father."