It is because 'LES' can be be used to refer to something specifically or generally. In this case it is used in the 'general' form - she knows (ALL) men i.e. she understands them generally, their behavior, motives etc. Because 'LES' is used generally, 'THE' is omitted. If it was used in a specific form it would have 'THE' in the sentence - She knows THE men i.e. she knows a specific group of men. Context will tell when it is specific or general, and here it is ambiguous that is why Duo allows the answer with and without 'THE' in it. But if Duo is picking one answer, you know from the translation they intend for this sentence to be in general (b/c it has no THE).....I know I am at a low french level, but I looked for the same answer many times and I am pretty sure I am correct
You are correct:
she knows a man - elle connaît un homme
she knows some men - elle connaît des hommes (plural of "un" = more than one)
she knows the man in the shop - elle connaît l'homme dans le magasin (specific)
she knows men and all their weaknesses - elle connaît les hommes et toutes leurs faiblesses (generality)
"She knows the men" is not more polite, but more specific, "the men" being those in the room or those mentioned before.
"She knows men" is general, where she knows "men" as a category, all and any of them.
Two separate meanings but one translation in French, since the definite articles "le, la, les" are used either for specific objects or for generalities.
3rd group, made of many verbs with varying endings in infinitive.
Indicative present = je connais, tu connais, il/elle connaît, nous connaissons, vous connaissez, ils/elles connaissent.
Past participle: connu, connue, connus, connues.
Same pattern for "paraître" (to seem), "apparaître" (to appear).