I think it's because in most of these exercises this 3rd person form is assumed to be 'he' or 'she'. If the formal 'you' is meant, usually the pronoun "Lei" would be included for clarity -- (though granted, here it could also be taken as 'she' since it'd be the first word in the sentence). In addition, the lower case "sue" would refer back to a 3rd person subject, not to "you" -- otherwise it'd be capitalized "Sue" (though I understand that capitalization for the formal 'you' isn't always obligatory.)
RamonAnaton: See my comment right above to Maateus or my comment to Brenda at the end. Without a clear subject, it can be He or She and without a spicific context it can be 'her keys or his keys" In other words it can be: He loses her keys or She loses his keys or any combination, even It loses its keys/his keys/her keys.
mercede777: the answer shown above is "She loses her keys." That said, "perde' could be he, she, or it. "le sue..." could be 'his, her, or its" Which answer is most logical depends on context, "it" being admittedly the least logical, but grammatically possible, as e.g. if referring to a rental company, real estate firm, automobile agency, etc.
"Lost" is past tense. Right now we're just learning the present tense.
"Lost" would be "perdeva".