A US correspondent writes to ask if I've come across modern dialect variations in if/then clauses, especially in Early Middle English. He cites If you go and I'll go meaning 'If you go, then I'll go' and biblical-sounding cases such as If he should command the stone to move, and it will move.
A linking adverbial use of and is certainly attested in Early Modern English, with a range of meanings such as 'therefore, and so, then' - an example from Shakespeare is 'Tis a good dullness, And give it way' (Tempest). I can't think of an instance after an if-clause, offhand - maybe a reader of this blog will dig one out.
These conjunctions shifted meaning a lot in Middle English, so that there are all kinds of overlaps. For instance, the overlap in meaning between if and and is well attested in Early Modern English, especially in initial position: 'An't be not four by the day, I'll be hanged' (Henry IV Part 1), 'I'll tell you when, and you'll tell me wherefore' (Comedy of Errors). Compare also 'so it please you / if it please you / and it please you', which we find throughout the period - as early as 1205, according to the OED.
I wouldn't be at all surprised to find the usage still around in modern English dialects. Uses of and as a subordinating conjunction have some interesting parallels with Celtic languages. The first place I'd look would be Irish English, where you can still hear today such sentences as There was lots of land for sale and I a young lad meaning 'when I was a young lad'. Anyone heard an if... and construction out there recently? Or anywhere else?