exsto

Exsto, exstare, exstiti is first conjugation, ex- plus sto, and it is defective at the far end: there is no supine in ordinary use, so the principal parts stop at three. Manuscripts spell it exto as often as exsto, and both are correct.

The literal sense is physical, to project above something, and it takes supra or ex with the ablative. The sense that earns it a place here is the historian's and the archivist's: "still exists, is on record, has come down to us". That is where English extant comes from, and it explains why the corpus overwhelmingly uses the third person forms exstat and exstant. Treat it as near-impersonal in practice; a first person exsto is grammatically fine and vanishingly rare.