"when you have finished" vs "when you finish"

[Eileen]

the students will be required to hand in their papers when they__________writing them.
1) hava finished 2) finish

what is the difference between "have finished" and "finish"?

i think "have finished" is more appropriate in that sentence. but can "finish" also use there?

[Alan]

Hi Eileen,

'Have finished' gives the idea of completing a process that has lasted a while. 'Finished' suggests simply 'stopped.'

Alan
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[Eileen]

thank you for the reply. now i undersrand.

Eileen