English subjunctive

[Jairo]

I heard it's dying and studied the basics of it. I just would like to ask about some examples (what is informal, affected, archaic or just plain wrong).

"If music be the food of love, play on." I heard this once. Was the present conditional expressed by the subjunctive some centuries ago?

"I'm doing this so he will see." Is it fine to use the future in purpose clauses?

"It was important that he do this." Can a past event be expressed by the present subjunctive?

Thanks!