…I might begin with the excellent Wikipedia article on the issue, which provides a nice introduction to its usage in English and other Indo-European languages.
I’m a native English speaker, but the first language I studied was Spanish, which I think accounts for my fascination with the subjunctive mood. It’s much more commonly heard in Spanish, partly because its usage in English disappears amid polysemy:
→ I wish you were nicer to your brother. (past subjunctive)
→ You were lucky he didn’t hit you. (past indicative)
Different words, but you wouldn’t know it. The only time you notice the subjunctive in English is when the verb doesn’t seem to match the subject:
→ If I were rich, I’d have you killed. (contrafactual)
→ I request that he be given exile. (indirect command)
→ Let us fight our enemies, not each other. (hortatory)
When the subjunctive shows up, there’s almost always drama. Someone is expressing hope or doubt. It’s worth paying attention.
Cynics have been predicting the death of the subjunctive for years, arguing that it is mostly confined to archaic phrases. I disagree. While there are many shaky grammatical constructs I could easily see collapsing (who/whom, lay/lie), I think the subjunctive has several points in its favor:
Most native speakers don’t know they’re using it. While we notice when it’s omitted (“If I was president…”), the majority of people get it right without knowing why. (“I demand my account be reactivated immediately.”)
While there are alternatives, they’re rarely better. The previous example could be rewritten, “I demand you reactivate my account…” or “Reactivate my account, you idiot!” But neither achieves the same effect as the subjunctive. English thrives on having many ways of saying similar-but-different things.
It’s really common in religious material. The U.S. is very church-y, so Americans get a weekly dosage of subjunctive in their sermons and prayers. (“The Lord bless thee and keep thee. The Lord make his face to shine upon thee. The Lord lift up his countenance upon thee and give thee peace.”)
Remember that the subjunctive is invoked by the semantics, not just the words leading up to it:
→ If I were correct with my answer, I would have won Jeopardy.
→ If I was correct in my calculations, we should hear a boom in three seconds.
Now that I’ve expressed my deep affection for the subjunctive, let me urge discretion when using it in screenwriting. Many times, your characters will speak ungrammatically. Your knowledge of the subjunctive should never trump their ignorance.
If was a bettin’ man, I’d say he demanded Sonny kills that other fella lest he rats him out to Bubba.
That’s three missed opportunities to use the subjunctive, but it may be the right choice for Pappy. Always go by ear with dialogue.