How long will a man lie i' the earth ere he rot?
The quoted line is Hamlet's. I wonder why the "rot" is not "rots".
"he rot" is the present subjunctive in this sentence. So the form is that of the bare infinitive. Another example, "Murder, though it have no tongue, will speak."
French uses subjunctive after avant que (before/ere + clause) still today and I assume that Shakespeare in some uses of the subjunctive is influenced by French subjunctive uses. Shakespeare's line is in Hamlet 5.1. Editions with annotations should explain why Shakespeare uses the subjunctive rot.
In Shakespeare-online.com I studied Hamlet 5.1. There are about 200 annotations but there is no annotation on the Sv (subjunctive) of "ere he rot".
I also found something on Shakespeares's grammar, but the paragraph on Sv is rather vague. It says nothing but that Shakespeare made more use of the Sv than is usual today. Shakespeare Grammar