"Ĉu vi partoprenos la junularan kongreson?"

Translation: Will you participate in the youth congress?

[JohnReid8]

This sentence literally looks like it means "Will you participate the Youth Congress?"

Is this a Esperanto idiom? I thought that Esperanto was supposed to avoid idioms when the idea could be expressed in an unidiomatic way.

[zerozeroone]

You can use a preposition or a direct object.

Looking at tekstaro.com:
...la princo ne partoprenis en tio. = ...the prince did not participate in that. [from La Faraono]
— Sed nia tuta rondo partoprenas ja la kunvenojn, insistis Johano. = — But our whole circle is participating at the meetings, instisted John. [from Infanoj en Torento]
(Not sure why it is kunvenojn in this case, as there isn't any motion. Typo?)

...kiuj partoprenas la kunvenojn. = who participates in the meetings. [from Infanoj en Torento]
Lian 6-partan simfonion partoprenas orkestro, ĥoro kaj apartaj kantistoj (la 3-a nivelo). = An orchestra, chorus, and special singers (3rd level) are participating in his 6-part symphony. [Kien fluas roj’ Castalie]
(A case of Obect-Verb-Subject usage in Esperanto, unless I'm totally confused again...)

Ni ankaŭ partoprenas! = We also are participating! [from Fabeloj de Andersen 4]

[JohnReid8]

It's still Volapük to me.

[kmile1]

The Esperanto sentence is just as non-idiomatic as the English one.
The only difference here between the Esperanto sentence and the English one is that English uses a preposition ('in', and not 'to', as I wrongly guessed ^^ because in French we say "participer à"), whereas in Esperanto there is no need for a preposition, you can simply, directly say "I participate something".
Now, what Zerozeroone is saying, is that if you feel like using a preposition, you have that possibility as well. Among the examples he/she gives, appears "partopreni en io" ('en': in).

[kmile1]

I think 'kunvenojn' is at the accusative form because it is a direct object.
'Ja' doesn't mean 'at', but "surely, indeed, rather, certainly" (according to Lernu's dictionary), so it doesn't count as a preposition.

[daguipa]

How can possibly using a direct object instead of a prepositional one be "an idiom"? Is everything that's different from your native language "an idiom" for you?

[Revilo_N]

Laŭ PIV oni povas uzi la akusativon kun "partopreni" anstataŭ la prepozicion "en", se oni uzas "partopreni" en la senco de "esti unu el tiuj, kiuj kune faras ion". Se oni uzas "partopreni" en la senco de "preni sian parton de io", oni devas uzi la prepozicion "en" kaj ne povas anstataŭigi ĝin per la uzo de la akusativo.

[tuxayo]

Varbado por la IJK jam komencas antaŭ la fino de la kurso ^^

Pli da informoj tie:
https://eo.wikipedia.org/wiki/Internacia_Junulara_Kongreso

http://tejo.org/cause/internacia-junulara-kongreso/