"I will do that under one condition."

Translation: Mi faros tion sub unu kondiĉo.

[Sunalooch]

I'm wondering whether the preposition "sub" makes sense here. It sounds not clear and not logical for me because my mother tongue is not English. I would use "kun unu kondiĉo" because that is how we say it in Russian and it seems more logical to me. Don't know, maybe it sounds same wrong for English Esperanto speakers us "sub unu kondiĉo" sounds to me. The point is that in Esperanto we should use the most suitable preposition based on logic and not on the mother tongue. Maybe someone can convince me that "sub unu kondiĉo" can make sense, for me "sub" sounds like a preposition describing position.

[lingvulo]

I agree with you, that "sub unu kondiĉo" doesn't sound like Esperanto (and I'm a native speaker of English). But I don't know what preposition people usually use here. I looked up kondiĉo in PIV out of curiosity and found a few interesting citations from Zamenhof (bold added):

1. "sub la kondiĉo de alpago oni konsentis sendadi al ŝi tagmanĝojn"
2. "kun la samaj kondiĉoj, kun kiuj laboras ĉiuj ĉi tiuj sinjorinoj"
3. "li ellernos nian lingvon kun la kondiĉo, ke ni elĵetu la vorton «kaj»"

So it looks like Zamenhof used both, though I think the third example, with "kun la kondiĉo", is quite parallel to the case at hand and sounds more "international" to me.

[elements2015]

Maybe it is taken from the German "unter der Bedingung" (unter = sub, der = la, Bedingung = kondicxio). In German this is an idiomatic expression. So, there it is.
That being said, although being German, logically I'd chose "kun". Makes more sense to me.

[kumko]

also in Polish you say "pod warunkiem" what means literally "under condition" (sub kondiĉo)

[amuzulo]

Thanks! I just added "kun" as an alternative "best" translation.

[JohnMoser1]

why 'je' instead of 'sub'?