Do thyself no harm (mēden praxēis seautōi kakon). The usual construction (mē and the aorist subjunctive) for a prohibition not to begin to do a thing. The older Greek would probably have used poiēsēis here. The later Greek does not always preserve the old distinction between poieō, to do a thing, and prassō, to practice, though prassete keeps it in Phi 4:9 and poieō is rightly used in Luk 3:10-14. As a matter of fact prassō does not occur in Matthew or in Mark, only twice in John, six times in Luke’s Gospel, thirteen in Acts, and elsewhere by Paul.
Sprang in (eisepēdēsen). First aorist active of eispēdaō, old verb, but here only in the N.T. Cf. ekpēdaō in Act 14:14. The jailor was at the outer door and he wanted lights to see what was inside in the inner prison.