"The judge locked the container."
Translation:La juĝisto ŝlosis la ujon.
I don't get the difference between the suffix -isto and -ulo...
Why is "jugxulo" not possible?
Or why "bonisto" is not possible?
-ulo relates to a person with that property (blindulo = blind person).
-isto relates to a person with that profession (or very regular occupation).
That's why your proposed words would not make too much sense. "juĝulo" would be a person with the property to be a verdict, "bonulo" a person with the profession to be good.
As I understand it, -isto indicates a specialist, while -ulo indicates your average joe. -isto is usually applied to some occupations, sports & hobbies, rather than adjectives. If a person specialised in being good, they'd be a bonisto, while a judge specialises in judgement, hence jugxisto. A jugxulo would be a regular judging guy (like that guy at work who's always criticising people, maybe?) (intermediate Esperanto, not fluent, not an expert, corrections by more experienced Esperantistoj welcomed)
-isto is also used for something that you do in a professional manner, like Esperantisto
And "sxtelisto"? Could it be sxtelulo?
I don't know... is stealing a profession?
You can steal in a professional manner in which case it would be ŝtelisto or you can have someone who steals which would be ŝtelulo. I imagine ŝtelisto would be much more common.
Professional manner? I'm certainly not an Esperantisto then!
"Mi estas Esperant-aĉ-isto" Or is: "Mi estas Esperant-et-isto" better.
"Mi estas krokodilisto?"
"fermi" is only "close". You can close the container without locking it, so, yes, "fermis" is incorrect here.
Ŝlosi also has a nuance of "sealing" something. An act a judge is more likely to accomplish than merely locking a box.