Learn Hardcore Japanese: toutyakuzikan ga kawattara, sugu renrakusite kudasai. - If the arrival time changes, please contact me right away.

Because 変わる is an intransitive verb meaning “to change,” the thing that changes is marked by が (the subject) inside the conditional clause. 到着時間が変わったら = “if the arrival time changes.” Using は inside this subordinate clause is generally avoided. You could topicalize outside the clause, e.g., 到着時間は、変わったらすぐ連絡してください。, but the given sentence with が is the most natural.

What’s the difference between 変わる and 変える here?

変わる (intransitive): “to change” (by itself). Example: 到着時間が変わったら…
変える (transitive): “to change (something).” Example: 到着時間を変えたら… = “if you change the arrival time, …” The original uses 変わる because it describes the arrival time changing (regardless of who caused it).

Does たら mean “if” or “when” in this sentence?

It can mean either, depending on context. With a request like this, it is usually understood as “if/when that happens, then do X right after.” The nuance is conditional (“if it happens”) but also implies the sequence “A happens, then B.”

How is たら different from なら, と, 時, or 場合?

たら: Natural for one-time events with a follow-up action. Best here.
なら: Assumptive; “if it turns out that…” Slightly more hypothetical. Example: 到着時間が変わるなら、すぐ連絡してください。
と: Used for natural results/general truths; not used with requests/commands. So × 到着時間が変わると、すぐ連絡してください sounds wrong.
時: Focuses on the time point. 到着時間が変わった時は、すぐ連絡してください。 is fine.
場合: Formal/official. 到着時間が変わった場合は、すぐにご連絡ください。

Do I need もし at the start?

No. もし is optional and adds emphasis to the “if.” もし到着時間が変わったら、すぐ連絡してください。 is fine but not required.

Why is there no particle after すぐ? Should it be すぐに?

すぐ is an adverb and doesn’t need a particle. すぐ and すぐに are both correct; すぐに can sound a bit more formal/emphatic. Extremely formal/urgent alternatives include 直ちに or 速やかに, but those are business/formal.

Can I put すぐ at the end, like 連絡してください、すぐ?

That sounds unnatural. Place it before the verb or right after the comma: …、すぐ連絡してください。 or …、すぐに連絡してください。 You can also say 連絡はすぐにしてください。 to emphasize “immediately.”

Why is 連絡して used before ください?

Because Vてください is the standard polite request pattern: “please do X.” Here, 連絡して (the て-form of 連絡する) + ください = “please contact.” You’ll also see it written together as 連絡してください (same meaning).

Is 連絡ください acceptable?

Yes, 連絡ください (dropping して) is common, especially in emails/texts. It’s succinct but a bit brisk. More formal is ご連絡ください, and the most neutral polite is 連絡してください.

What about 連絡をする or 連絡をしてください?

They’re correct but slightly stiffer. 連絡する is more common in speech. Polite options: 連絡してください, 連絡をしてください, or more formal ご連絡ください.

Do I need to specify whom to contact?

Context usually makes it clear. If needed, add a recipient:

私に連絡してください。 (contact me)
こちらにご連絡ください。 (contact us/this office)
この番号に連絡してください。 (contact this number)

Is this polite enough for business?

It’s standard polite. For customer-facing or very formal contexts, upgrade to:

到着時間が変わりましたら、すぐにご連絡ください。
到着時間に変更がございましたら、すぐにご連絡くださいませ。
Softer ask: ご連絡いただけますか。/ ご連絡いただけますと幸いです。

Are the spaces and punctuation in the given sentence normal in Japanese?

In normal Japanese, there are no spaces. The standard punctuation would be: 到着時間が変わったら、すぐ連絡してください。

Should it be 到着時間 or 到着時刻?

Both are used. 時刻 is the precise clock time; 時間 can mean “time” or “duration.” For schedules, 到着時刻 (or 到着予定時刻) is more precise/formal. In everyday speech, 到着時間 is very common.

How do I read the words?

到着時間: とうちゃくじかん
変わったら: かわったら
すぐ: すぐ
連絡して: れんらくして
ください: ください

Is there a common synonym using 変更?

Yes. Very common in business:

到着時間に変更があったら、すぐにご連絡ください。
Even more formal: 到着時間に変更がございましたら、すぐにご連絡ください。