A song lyrics got me thinking.
If you leave me I'll miss you much.
I know songs are absolved of following strict grammar thus I'm not criticizing it. My question is as follows: what tense(s)* are appropriate for expressing 'if/then in the future'.
Follow all the modifications of that sentence which sound at least somewhat OK in my head.
<the original>
If you left me I would miss you much.
If you leave me I would miss you much.
If you leave me I'm gonna miss you much.
If you have left me I'd be missing you much.
* - in the popular meaning, not the linguistic science's totally useless one