Let’s take a look at the three main categories of such words:

1. Words that name the people who do a certain kind of work. For example:

pastro = pastor
advokato = lawyer
arkitekto = architect
biologo = biologist
kirurgo = surgeon
konduktoro = conductor
profesoro = professor
reĝo = king
aŭtoro = author
studento = student

For these words, the suffix -ist- isn’t used. It creates a redundant form. Biolog/ist/o, for instance, would mean “a biologist-ist”. Kirurg/ist/o would mean “a surgeon-ist”. And those simply aren’t concepts that people use.

2. Words that are formed from roots that name something other than the people who do the work or activity, using -ist-. For example:

instrui – instruisto = teacher
esplori – esploristo = researcher, explorer
baki – bakisto = baker
biciklo – biciklisto = bicyclist
poŝto – poŝtisto = postal worker
verki – verkisto = writer
juĝi – juĝisto = judge
kultivi la teron – terkultivisto = farmer

3. Words formed using the participle -ant- or -int-. For example:

lerni – lernanto = pupil
helpi – helpanto = helper
krei – kreinto = creator

Note that sometimes two or more different forms are used. For example:

traduki – tradukisto = a (professional) translator
traduki – tradukanto = a person translating something
traduki – tradukinto = a person who translated something