Look thy last on all things lovely, Every hour. Let no night Seal thy sense in deathly slumber Till to delight Thou have paid thy utmost blessing.
-Walter de la Mare
Grayling claims that Thou have is an error, and one committed by the poet. It should, he says, be Thou hast. But Alan Larman, of Congleton, Cheshire, UK, writes to NewScientist that this is the subjunctive mood and no error at all.
In modern English, the use of the subjunctive is highly restricted and occurs mainly in subordinate clauses introduced by that (e.g., It's important that he be on time; I take were in if I were you to be an isolated irrealis form and not part of the subjunctive, which uses the plain form of the verb). My problem is this: I don't know enough about the grammar of early modern English (the style, if not the time, in which la Mare was writing) to know whether the subjunctive is possible here. The poet was writing in the early part of the twentieth century, at which time thou had been out of use for perhaps 250 years. So it is indeed possible that, in trying to be poetic, he goofed.
There is, however, a still-used frozen form that displays exactly the pattern in question: until death do us part, which suggests strongly that the error is Grayling's.
If anybody can settle this definitively, I'd appreciate hearing from you.