What to do?

I happened across an interesting little paragraph in the PMEG! Check out the paragraph that starts

ĉefverbecaj I-verboj aperas ankaŭ en iaj mallongigitaj esprimoj de dubo aŭ hezito.
mainverb-like I-verbs also appear in some shortened expressions of doubt or hesitation.

So normally the “-i” form of a verb isn’t used as the main verb in a sentence; it doesn’t have any mood or tense of its own. The “-i” form is the infinitive, the base form:

ami 
to love

kuri
to run

fajfi
to whistle

And when you want to use them in an appropriate tense/mood, you alter them:

mi amas Esperanton 
I love Esperanto

kuru! 
run!

li fajfis la tutan tagon
he whistled the entire day

But one of the ways in which I-verbs do occur as the main verb is in these expressions of doubt or hesitation. And they’re normally shortened versions of what you could express, say with a “u-verb” (see previous post). Take the example from that PMEG page:

kion fari? 
what to do?

Which is like a short form of:

kion mi faru? 
what should I do?

See how we kinda do it in English too in the translations? The usage is very conversational. So:

“Savu nin!”, “Sed la meloj estas nevenkeblaj! Kiel mi povas savi vin? Peti ilin ĝentile?” 
“Save us!”, “But the badgers are invincible! How can I save you? Ask them nicely?”

So peti would be a main verb there, all by itself. Rather than the full version:

ĉu mi petu ilin ĝentile?
should I ask them nicely?

And that’s how I understand the usage… You can see how it changes in English too.