Once + past form of the verb?

[Yura Reiri]

I'm still hesitant about using 'once' because I don't know what form of the verb will follow.

Example:

Call me once you finished it.

Call me once you finish it.

Which is correct?

Thank you.

[Raymott]

I'm still hesitant about using 'once' because I don't know what form of the verb will follow.

Example:

Call me once you have finished it.

Call me once you finish it.

Which is correct?

Thank you.

The first one isn't correct unless you make that correction. The amended sentence is in the present perfect, and the second is in the simple present. They are now both correct.

[Yura Reiri]

So I can only use it with the simple present and present perfect. Thank you.
But what will be the difference in using the simple present and the present perfect?

[Raymott]

So I can only use it with the simple present and present perfect. Thank you.
But what will be the difference in using the simple present and the present perfect?

In practice, there's no difference in meaning.