"We reached our goal."
Translation:Ni atingis nian celon.
PIV defines trafi as Atingi per ĵetita objekto tion, kion oni celis.
So it's specifically for thrown things, while atingi can be either mean reaching something by one's own motion or by throwing something.
So "trafi celon" would be something like "hit the target" (with an arrow or a bullet that you shot) while "atingi celon" could mean either "hit the target" (like with "trafi") or more generally "reach a goal, attain a target" more metaphorically, with no throwing or shooting involved.
Yeah well it's not only specifically for thrown objects, that is just one of 5 definitions.
Though generally if you don't trafi your goal, then you missed it.
The ways trafi is used:
Trafi du celojn per unu ŝtono - To trafi two goals with one stone.
La kuglo trafis lin meze en la frunto - The bullet trafis him in the middle of his forehead.
Hakilo trafos malmolan lignon - An axe will trafi hard wood.
Morto trafis ŝin - Death trafis her
La bovoj haltis, kiam ili trafis freŝan herbon - The cows stopped when they trafis fresh grass.
So generally speaking, had we used trafi here instead of atingi it would be understood as if someone threw something or randomly walked into it (which is hard if is not a actual physical goal.)
While with atingi it is implied, that we set forth a goal, made an effort and reached it.
Esperanto is more like Scandinavian or Romance than like Slavic in this respect -- si only exists for the third person.
So where a Russian might say the equivalent of Mi havas sian libron for "I have my (own) book" or Ni atingis sian celon, Esperanto has to use mian, nian, etc. since sian would only be for li ŝi ĝi ili.
Hardly! But yes, Esperanto has some arbitrary rules. Fortunately the rules are few, exceptions basically non-existent, very low memory load. By the way, if you happen to say something like "Mi atentigis sian celon", you would be understood. The sentence is non-standard but unambiguous, since "si" refers back to the subject of the sentence or clause. Ĝuu! / Enjoy!