I am beginning to think that the imperfect is perhaps one of the more subtle tenses, and one whose full significance is most often overlooked.
Sunday school last week was on Mark 8:14-21. Jesus just finished the Feeding of the 4,000, and as they are traveling across the lake he says, “’Be careful,’ Jesus warned (διεστέλλετο) them. ‘Watch out for the yeast of the Pharisees and that of Herod’” (NIV; cf. also NLT).
διεστέλλετο is imperfect, but there is no way someone would understand this from “warned,” which sounds more like an aorist. Other translations include “cautioned” (ESV, NRSV), “commanded” (HCSB), “ordered” (NET), and “charged” (KJV).