Learn Hardcore Japanese: kare ha kyuuyou ga dekita node, mukou no kaizyou ni ha ikenaikoto ni natta. - An urgent matter came up for him, so it has been decided that he can’t go to the venue over there.

It literally means “an urgent matter came into existence.” Here できる/できた isn’t “can/could,” but “to come about / be formed.” With words like 用事/急用, X ができた idiomatically means “something (urgent) came up.” Common alternatives: 急用が入った, 急な用事ができた, 都合が悪くなった.

Why is ので used instead of から? Are they interchangeable?

Both give a reason and are fine here. ので sounds softer, more objective, and is polite-friendly (often used in explanations/apologies). から is more direct and can sound blunt in formal contexts. So:

急用ができたので… (softer, explanatory)
急用ができたから… (more direct “because”)

What’s the role of には after 会場? Why both に and は?

に marks the destination; は topicalizes/contrasts. Together (には) they mean “as for (going) to that venue,” with a contrastive feel.

向こうの会場に行けない = can’t go to that venue (neutral).
向こうの会場には行けない = as for that venue, (he) can’t go (implying contrast, e.g., maybe he can go elsewhere).

Could I use へ instead of に here?

Yes: 向こうの会場へは行けない. へ emphasizes direction a bit more; に is more common with destinations. Without は, へ行けない is possible but に行けない is more idiomatic.

Why use the potential negative 行けない and not 行かない?

行けない = “cannot go” (inability due to circumstances).
行かない = “will not go” (choice/decision).
An urgent matter “coming up” causes inability, so 行けない fits.

What does ことになった add to 行けない?

V-plain + ことになる/なった frames the result as an arrangement/decision/outcome that has been reached, often by circumstances or others.
行けないことになった = “It turned out/was decided that he can’t go.” It sounds less like his personal choice and more like the situation has led to that conclusion.

How is 行けないことになった different from 行けなくなった?

行けなくなった: “became unable to go” (focus on change in ability because of events).
行けないことになった: “it ended up that he can’t go / it was decided he can’t go” (focus on the outcome/arrangement).
They overlap in casual cancellations, but if you want the neutral “I ended up unable to go,” 行けなくなった is often more straightforward.

What’s the difference between ことになった and ことにした?

〜ことになった: decided/has turned out (external factors or collective decision).
〜ことにした: I/we decided (speaker’s own decision).
So 行かないことにした = “I decided not to go.”
行けないことになった = “It’s been decided/I ended up not being able to go.”

Could I say 行かないことになった instead?

Yes, but it changes nuance. 行かないことになった = “it’s been decided he won’t go” (decision/policy). 行けないことになった = inability (constraint). Use the one that matches the situation.

Is 向こうの会場 “the other venue” or “the venue over there”? What does 向こう add?

向こう is deictic: “over there / the other side / the other one (relative to us).” 向こうの会場 can mean “the venue over there” or “the other venue” (in contrast to a nearer/current one). Politer: あちらの会場. More explicit location: 向こうにある会場. If you just mean a different venue without the “over there” nuance: 別会場.

Why 彼は and not 彼が?

は marks the topic (“as for him…”). The subject of できた is 急用 (marked by が), so 彼 isn’t the subject of that verb. 彼は sets him as the topic for the whole sentence.

Is the が in 急用ができた the “but” が?

No. It’s the subject marker, making 急用 the subject of できた (“an urgent matter arose”). The clause-linking “but” が comes between two clauses; that’s not what this is.

Can I omit 彼は?

Yes, if context makes the subject clear:
急用ができたので、向こうの会場には行けないことになった.
Japanese often drops pronouns; using a name/title is also common.

How formal is this sentence? How to make it more or less polite?

As written: neutral.

More polite: 彼は急用ができたので、向こうの会場には行けないことになりました.
Business-formal: 彼は急用が生じたため、向こうの会場には出席できないこととなりました.
Casual: 彼、急用できちゃって、向こうの会場には行けなくなった.

Why are both できた and なった in past? Is that “double past”?

Each verb’s tense reflects its event:

急用ができた: the urgent matter occurred (past).
行けないことになった: the state/outcome of “can’t go” was reached (resultative past that still holds now). Japanese commonly uses past to indicate a state that has been established.

Could I use 用事 instead of 急用?

Yes, but nuance changes:

急用 = urgent business (time-critical).
用事 = errand/engagement (not inherently urgent).
急用ができた is a fixed, idiomatic “something urgent came up.”

What is the potential form of 行く and why isn’t it 行かれる?

The potential of 行く is 行ける (godan → e-row). 行かれる exists but is the passive/honorific of 行く, not the potential.

Can I replace ので with ため?

Yes, for a more formal, written tone:
急用ができたため、向こうの会場には行けないことになった。
Rough guide: ため (formal/objective) > ので (polite-neutral/soft) > から (direct).

How can I explicitly contrast venues, like “he can’t go to that one, but he can go to this one”?

Use は on both destinations to mark contrast:
向こうの会場には行けないが、こちらの会場には行ける。

Is there a difference between ことになった and こととなった?

こととなった is a more formal/written variant often used in announcements. ことになった is the standard everyday form.