Why can't I use "was dead" here?

[Enlearnercn]

English Grammar Tests, Elementary Level

ESL/EFL Test #88 "Common English Errors (4)", question 1

Her mother ......... when she was just a little girl.

(a) is dead
(b) dead
(c) died
(d) was dead

English Grammar Tests, Elementary Level

ESL/EFL Test #88 "Common English Errors (4)", answer 1

Her mother died when she was just a little girl.

Correct answer: (c) died

Your answer was: correct
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Dear Linda,

I remember a line in Roman Holiday "When the doctor comes, I will be dead." Why cann't I use "was dead" here?

Thanks for your explanations before. You have helped me a lot.
Thanks again.

Enlearnercn

[Mister Micawber]

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The when-adverbial clause indicates a range of time, so that died-- an instantaneous activity within that time span-- is called for. Was dead on the contrary indicates an ongoing condition.
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Native English teacher at Mister Micawber's