More is not necessarily more
I realised after a while that I’ve been reading sentences containing pli and plu for a while now without any problem. But I translate them most often as “more” (both of them). It occurred to me that Esperanto usually has a good reason for having more than one of these little words despite there being a single English translation. I realised I had no idea what the difference was, so went on a mission to find out!
Pli versus Plu!
Abstractly, the difference is this: plu means “further/additionally/more” in relation to time or space. Pli means “more” but in relation to size, grade or degree.
So while plu is used for something that is continuing or ongoing, pli means “more than a certain amount” and, as such, is used for comparisons.
la melo ne plu aŭskultas al mi
the badger no longer [/more] listens to me
la melo estas pli bela ol la kato
the badger is more beautiful than the cat.
This shows that obviously if you use ne in conjunction with plu it means to not continue etc.
Something a little harder? I found this example on the PMEG page about plu using the word rakonti “to relate/tell/narrate”:
li rakontis plu
he continued his narration (narrated more)
li rakontis pli
he narrated more things (than previously/up until now/than others do/did)
Isn’t that awesome? With the change of a single character you get a whole different feel of the word “more”.
“Being Colloquial in Esperanto” also has a bunch of example sentences of the two words (alternatively, p.184 of the paper copy).
Bonus fun!
As pointed out in the comments of the beforetimes, sometimes another word can act as “more”. This word is ankoraŭ which most often is translated as “still/yet”. It describes a past action/state that is still in effect.
However it also means something else. I’ll give you an example from the PMEG page:
poste mi ankoraŭ parolos pri ĝi
afterwards I will still (more) talk about it. (I will add to what I’ve said)
poste mi plu parolos pri ĝi
afterwards I will talk more about it. (I will resume the same talk)
In these cases, ankoraŭ shows repetition or more of the same type of thing. Plu shows continuation of the same thing.