What is the rule for choosing coniunctivus or imperativus when expressing commands? I know, that imperativus has only second person forms, so one is forced to use coniunctivus for other persons. Are there other rules however that make sentences with these modes different?
Hmm. My understanding is that the bare subjunctive as a positive request/command is actually rather rare in classical Latin. Woodcock's New Latin Syntax, p. 97, after a discussion of noli, nolite + infinitive as more polite than ne + perfect subjunctive, tells us:
Besides noli, nolite with the infinitive, the following periphrases also are frequently employed for the sake of politeness: (i) For positive commands, fac, facite, with a quasi-subordinate subjunctive: Fac mihi scribas; facite mihi scribatis, "See that you write to me." The subordinating conjunction ut is usually omitted. Similarly vide, "see that . . .", and velim, "I should like . . ." are used with the subjunctive: Plaut Asin 755 scribas vide plane et probe, "see that you write clearly and properly"; Cic. Fam. 9, 12, 2 tu velim animo forti sis, "I should like you to be of brave heart." (ii) For negative commands fac ne . . ., facite ne . . ., and vide ne . . . are used. Rather more frequent are cave (sometimes caveto) and cavete with, or more often without, ne: Plaut. Most. 324 cave ne cadas, "mind you don't fall"; Cic. Att. 3, 17, 3 cave vereare, "mind you don't fear."
If you really want to give a command, use imperative. If you want to request someone to do something, use conjunctive. The difference between curre and curras is similar to the difference between "run" and "please run".