which is the correct one?

[Vivian123]

Hello, can anyone help me with this?

a) Since you are not using this pen anymore, can you please return it to me?
b) Since you have not use this pen regularly, can you please return it to me?
c) Since you have not been using this pen, can you please return it to me?

Is any of these correct? Otherwise, how should I say it?

 

[GoesStation]

A and C are correct. B would be correct if you replaced "use" with its past participle, used.

 

[Vivian123]

Dear GoesStation,

Correction: Since you have not used the pen regularly, can you please return it to me?

Thanks for your help.

By the way, will there be any difference if I use 'can' you or 'could' you? What is the difference?

 

[GoesStation]

By the way, will there be any difference if I use 'can' you or 'could' you? What is the difference?

Could you is a little "softer" and more polite, at least in AmE.

 

[emsr2d2]

"Could you return it to me?" is more like saying "I wonder if you would mind/if you would be able to return it to me".
"Would you return it to me?" is closer to "Please return it to me".

 

[andrewg927]

There is not much difference between "can" and "could" but as GS pointed out, there is this subtle difference.