Broadly speaking, we may say that present commands give general or repeatable instructions, while aorist commands give specific directions. There are numerous exceptions: for example, some verbs may take one tense or the other by convention, not by conscious choice; also, third person prohibitions favor the aorist tense five to one. If you are in doubt, simply translate Greek commands as English commands, and leave the rest to context. EXAMPLES: λογίζεσθε ἑαυτοὺς [εἶναι] νεκροὺς τῇ ἁμαρτίᾳ