past simple, past perfect - if only

[Lenka]

What a pity you didn't see the play.
= If only you had seen/could have seen/saw the play.

Why is it incorrect to use "saw" in the sentence? Would it refer to the present (What a pity you don't see the play.)?

[Harry Smith]

What a pity you didn't see the play.
= If only you had seen/could have seen/saw the play.

Why is it incorrect to use "saw" in the sentence? Would it refer to the present (What a pity you don't see the play.)?

Because "What a pity you didn't see the play" is a past action. When we speak about the past, If only is followed by past perfect.