I have learnt that there are 3 basic forms of imperative:
1) For तू: verb root जा! Go! बोल! Speak!
2) For तुम: root + -o जाओ! बोलो!
3) For आप: root + -ie/iye जाइये! बोलिए!
The infinitive can also be used as an imperative: जाना! बोलना!
I have found this link that provides more details about the imperative in Hindi: http://hindilanguage.info/hindi-grammar/verbs/mood/imperative-mood/
Stem withoutओ is not really taught here on Duolingo. It's a little strange to me to introduce it just in this one. Also, it's rare. Btw: This stem + ओ form like बोलो that Duolingo teaches, is not really polite. It's used for commands if people are your servant. Better is to use stem+इये (like बोलिये), which closer to please in English.