Por. Pro.
"Por" means "for, in order to, for the purpose of, for the benefit of",
as "Li acxetis inkon por skribi", He bought ink in order to write. Por
is one of the three prepositions used before verbs in the Infinitive.
"Pro" means "for, because of, on account of, for the sake of" (cause),
as "Li demandis sxin, pro kio sxi ploras", He asked her, for what
(reason or cause) she weeps.
"Por" looks "forward" to the aim or purpose of the action, to that which
comes after the action, while "pro" looks "back" to what came before
the action and caused it to be done, as "Mi mangxas" pro "malsato" por
"vivi", I eat because of hunger in order to live.