She will have written the letter by 5 o' clock.
She-- подлежащее
Will have written-сказуемое, Future Perfect Active.
by 5 o'clock---индикатор, указатель времени.
Она закончит писать письмо к 5 часам.
She won't have written the letter by 5 o'clock.
Will she have written a letter by 5 o'clock?
Yes,she will.