Can I use the following form:
Please tell me when you finished filling in this form.
Or does it have to be something along the lines of:
Please tell me when you have finished filling in this form.
Can I use the following form:
Please tell me when you finished filling in this form.
Or does it have to be something along the lines of:
Please tell me when you have finished filling in this form.
Your first sentence
Please tell me when you finished filling in this form.
is not correct, you could use either present or present perfect
Please tell me when you are finished filling in this form.
Please tell me when you have finished filling in this form.
alternatively you could use
Please tell me when you finish filling in this form.
All three mean "upon completion, please let me know".