Learn Hardcore Chinese: wǒ yǒu yì jiàn shìqíng yào gàosu nǐ.

In the pattern 有…要…, 要 marks intention/necessity and often corresponds to “to” in English: “have something to tell you.” It’s about an intended or imminent action, not a pure future tense marker.

Softer intention: 我有件事想告诉你 (I’d like to tell you)
Obligation: 我有件事得/必须告诉你 (I must tell you)

Why use 一件事情? What’s the difference between 事, 事情, and 事儿? And why the classifier 件?

事 and 事情 are near‑synonyms; 事 is shorter and more colloquial; 事情 can feel a bit fuller/more formal.
事儿 (shìr) is the northern “er‑hua” spoken form of 事, very colloquial.
The standard classifier for 事/事情 is 件: 一件事 / 一件事情.
In informal northern speech you may hear 一个事, but 一件事(情) is more broadly standard and natural. 一个事情 is understood but sounds less idiomatic.

How should 一 be pronounced in 一件? Is it yì or yí?

Because 件 (jiàn) is fourth tone, tone sandhi changes 一 to second tone: say yí jiàn. Rule of thumb:

Before a 4th‑tone syllable: 一 → yí (2nd), e.g., yí gè, yí jiàn
Before 1st/2nd/3rd tones: 一 → yì (4th), e.g., yì tiān, yì nián, yì wǎn
When emphasized as the number “one”: stays yī (1st)

Is 告诉 pronounced gàosu or gàosù?

For “to tell,” the common Mainland pronunciation is gào·su (neutral tone on the second syllable). You may also hear gàosù (4‑4), especially in Taiwan. There is also a formal/legal sense of 告诉 (gàosù) meaning “to bring a (private) complaint,” but in everyday “tell,” the neutral second syllable is standard.

Why is it 告诉你 and not 对你说? Are 告诉 and 说 interchangeable?

告诉 takes a person directly: 告诉你.
说 does not; you need a preposition: 对你说 / 跟你说.
Nuance: 告诉 often implies conveying definite information; 跟你说/对你说 can feel more conversational. All of these are fine here: 我有件事要告诉你 / 我有件事要跟你说.

Should there be a 的, like 我有一件要告诉你的事情?

Both are correct, with different structures:

我有一件事情要告诉你 uses “有 + 名词 + 要/想 + 动词” (post‑modifier).
我有一件要告诉你的事情 uses a pre‑posed modifier with 的. The 的 version slightly highlights “the thing (that I want to tell you).” The no‑的 version is very common in speech.

Can I say 我要告诉你一件事情 instead? Any difference?

Yes. 我要告诉你一件事情 emphasizes your intention to tell. 我有一件事情要告诉你 emphasizes that there exists a matter. Both are natural.

Is 要 necessary? What about 我有一件事情告诉你?

我有一件事情告诉你 is fine. Without 要 it sounds a bit more neutral/factual. With 要 it foregrounds the plan to do it. Very common alternatives:

我有件事跟你说
我有话要对你说

Can I drop 一 or even the classifier?

Dropping 一 is very common: 我有件事要告诉你.
You can also drop the classifier and treat 事 as “something”: 我有事要告诉你. Very idiomatic.

What order do the person and the content take with 告诉? Can I say 告诉一件事情你?

The person follows 告诉 directly:

Correct: 告诉你一件事情 / 告诉你这个消息
Incorrect: 告诉一件事情你 You can also topicalize the content: 这件事我要告诉你.

Why not 对你告诉 or 告诉给你?

They’re nonstandard. Use:

告诉你 (no preposition)
Or with 说: 对你说 / 跟你说 Regional speech may have 给你说, but standard Mandarin prefers 跟你说. Avoid 告诉给你.

Can I use a 把 sentence, like 我要把这件事告诉你?

Yes: 我要把这件事告诉你 is natural. With 把, the object should be specific/definite (这/那/那件). Using an indefinite 一件事 with 把 is usually avoided.

Are there other nouns I can use instead of 事情?

Yes, pick according to meaning:

话: 我有句话要对你说 (something to say)
消息: 我有个消息要告诉你 (a piece of news)
秘密: 我有个秘密要告诉你 (a secret)

When should I use 您 instead of 你 here? What about plural?

Use 您 for politeness/respect: 我有件事要告诉您. For plural, use 你们: 我有件事要告诉你们.

Do I need 了 if I’m about to tell the person now?

Not required. Adding 了 can mark a “now it’s happening” shift: 我有件事要告诉你了. To say it already happened: 我已经告诉你了.

Any small stylistic softeners for politeness?

Use softeners like 一下/一声/先:

我有件事想跟你说一下 (a quick word)
我先跟你说一声 (just to let you know first)