一段 verbs may be conjugated to form imperative forms of ～よ or ～ろ.
For example:
How did the two forms come about?
What is the difference in usage of the imperatives?
When will one form be used over the other? Or what determines the preference?
How did the two forms come about?
The original imperative did not end in -yo or -ro. In Old Japanese, it took the following form: upper monograde: -i1, upper bigrade: -i2.† To this, an emotive -yo may be added, but it was optional.
During the transition to Early Modern Japanese, the 上代特殊仮名遣い distinction 1/2 was lost. This resulted in confusion between the irrealis and imperative for mono/bigrade verbs, which both now ended in either -i or -e. The solution for this was to require the emotive suffix -yo to the end of the imperative, which then became mandatory for mono/bigrade verbs and as such is now inseparable.
The -ro suffix found in volumes 14 and 20 of the Man'yōshū, which are the chapters covering the eastern dialects. Usage is identical to western -yo. Like emotive -yo, an emotive particle -ro may also be found. (cf #3552)
What is the difference in usage of the imperatives? When will one form be used over the other? Or what determines the preference?
With the propagation of the Tōkyō dialect, -yo forms sound old and even formal. As such, you will typically find the -yo forms in written materials (test instructions etc) while the -ro forms in spoken discourse.
†Monograde (一段) is a verb conjugation type characterized by the irrealis (未然形) and adverbial (連用形) both ending in -i (上) or -e (下). The 1/2 (甲乙) notation indicates two different phonologically sounds. For example, there were two types of ki: /ki1/ and /ki2/. One phonetic reconstruction for this *ki and *kɨ. (The * indicates a reconstruction.) This difference cannot be be written in kana. Look up 上代特殊仮名遣 for further details.
