Learn Hardcore Chinese: wǒmen jiǔ diǎn chūfā ba.

吧 softens the sentence into a friendly suggestion/invitation, roughly “Let’s …” or “Shall we …”. Without 吧, 我们九点出发 is a plain statement (“We leave at nine”). With 吧, it invites agreement rather than imposes a plan.

How is 吧 different from 吗 or 好吗 here?

吧: proposes/softens (Let’s leave at nine).
吗: asks for confirmation (Are we leaving at nine? — 我们九点出发吗？).
好吗/行吗: explicitly seeks approval (Let’s leave at nine, okay? — 我们九点出发，好吗？).

Is the word order “subject + time + verb” the normal order?

Yes. A very common pattern is: Subject + (time) + (place) + verb.

Natural: 我们九点出发吧。
Also fine (emphasis on time): 九点我们出发吧。
Unnatural: 我们出发九点吧。 (time shouldn’t trail the verb like this)

Should I add 在 before the time (like “at nine”)?

No. For clock times, Chinese typically uses the time phrase by itself:

Natural: 我们九点出发吧。
With 在 is not needed and often sounds stilted in everyday speech here. Save 在 for locations or certain formal frames (e.g., 在九点之前, “before nine”).

Do I need 钟 after 点? What’s the difference between 九点 and 九点钟?

Both are correct on the hour:

九点 is the everyday default.
九点钟 can sound a bit more explicit/formal or when stating a time in isolation. In scheduling talk before a verb, 九点出发 is usually preferred.

How do I say other times like 9:15, 9:30, or “about nine”?

9:15: 九点十五分 or 九点一刻
9:30: 九点三十分 or 九点半
About nine: 九点左右（e.g., 我们九点左右出发吧）
8:55: 八点五十五分 or 差五分九点

Any pronunciation tips (tones, sandhi) for this sentence?

九点 (jiǔ diǎn): third + third; in flow, 九 changes to second tone: jiú diǎn.
我们 (wǒmen): 们 is neutral tone.
出发 (chūfā): both first tone.
吧 (ba): neutral tone. So a natural flow is: wǒmen jiú diǎn chūfā ba.

What’s the nuance difference between 出发, 走, 离开, and 出门?

出发: “set off/depart,” often for a trip or planned departure.
走: “go/leave” (very colloquial and general). 走吧 = “Let’s go.”
离开: “leave (a place/person),” more formal/neutral, not specifically trip-related.
出门: “go out/leave home.” If you mean the start of a journey, 出发 is the best fit.

Is 出发 transitive? How do I add origin and destination?

出发 is intransitive by itself. Add origin/destination with prepositions:

Origin: 从 + place + 出发 (我们九点从酒店出发)
Destination: 出发去 + place (我们九点出发去机场) You can combine: 我们九点从酒店出发去机场。

Can I drop 我们?

Yes. 九点出发吧 is natural if “we” is understood from context. For extra clarity that the listener is included, northern Mandarin often uses 咱们: 咱们九点出发吧 (“we, including you”).

How do I make this firmer or softer?

Plain plan: 我们九点出发。
Soft suggestion: 我们九点出发吧。
Seek approval: 我们九点出发，好吗/行吗？
Offer a choice: 我们九点出发还是十点出发？
Strong directive: 九点出发。

Should this end with a period or a question mark?

With 吧 used as a suggestion, a period (。) is standard: 我们九点出发吧。
In casual texting, people sometimes use 吧？ to signal rising intonation, but the neutral written form takes a period.

How do I specify AM/PM?

Add a time-of-day word:

Morning: 上午九点/早上九点
Afternoon: 下午九点 (rare; usually means 9 p.m. is 晚上九点)
Evening/night: 晚上九点 Example: 我们晚上九点出发吧。

Is it okay to write 我们于九点出发吧?

Mixing 于 (formal “at”) with 吧 (colloquial) clashes in register. Use either:

Formal: 我们于九点出发。
Colloquial: 我们九点出发吧。

How do I suggest not leaving at nine?

Use 别/不要 for a negative suggestion, then propose an alternative:

我们九点别出发，太早了。改成十点吧。
不要九点出发，我们十点走吧。

What nuance do 就/再/才 add with time?

就: earlier/sooner than expected: 我们九点就出发吧 (“Let’s head out as early as nine.”)
再: wait until then / not before then: 我们九点再出发吧 (“Let’s leave at nine, not earlier.”)
才: “not until,” mainly used in statements: 我们九点才出发 (“We don’t leave until nine.”). It’s uncommon in a proposal with 吧.

Why are there spaces between characters here?

They’re just for teaching segmentation. In real Chinese writing you’d write: 我们九点出发吧。