Learn Hardcore Chinese: wǒ ràng tā zài wàimiàn děng wǒ.

让 + person + verb phrase is a causative: “have/make/tell (someone) to do something.” In everyday Mandarin it most often means “tell/ask” or “have,” not simply “allow.” For example: 老师让我们做作业。 = “The teacher told us to do homework.” In your sentence, the default reading is “I told/had him wait for me outside.”

Is 让 polite? If I want to be more polite, what should I use?

让 can sound neutral to somewhat commanding depending on context and relationship. To sound politely requestful, use 请: 我请他在外面等我。 To stress permission rather than instruction, use 允许: 我允许他在外面等我。

Can I use 叫 or 使 instead of 让? What’s the difference?

叫 is a near-synonym in speech: 我叫他在外面等我 (very natural). 使 means “to cause” and is formal/literary, better with adjectives or abstract results, e.g., 这件事使我很难过。 Using 使 in your sentence sounds stiff.

Why is 在外面 before 等? Could I say 我让他等我在外面?

Place/time phrases typically go before the verb they modify. 我让他在外面等我 is correct because 在外面 modifies 等. 我让他等我在外面 is unnatural/wrong; 在外面 doesn’t attach properly there.

Do I need 在? Can I say 我让他外面等我?

Yes, you need 在 to mark a location for an action. 我让他外面等我 is ungrammatical. If you mean “go outside and then wait,” use 到外面（去）等我.

What’s the difference between 在外面 and 到外面?

在外面: the action happens at that location (wait outside).
到外面（去）: movement to a destination first, then the action.
For “Go outside and wait for me,” say 我让他到外面去等我.

Does 等 require an object? Why is 等我 necessary?

When you specify the thing/person waited for, 等 takes an object: 等我/等车/等通知. Without an object you usually add a time word or 着: 在外面等一下 / 等一会儿 / 等着. Dropping 我 changes the meaning to “wait (unspecified).”

What does 等着 add compared with 等?

等着 emphasizes an ongoing, continuous state (“keep waiting”). 我让他在外面等着我 implies “keep waiting for me outside (don’t leave).” Plain 等 is neutral about continuity.

Could I use 等候 or 等待 instead of 等?

Yes, but they’re more formal/literary. 我让他在外面等候我/等待我 is correct but sounds official; everyday speech prefers 等我.

Can I drop the second 我 and just say 我让他在外面等?

Not if you mean “wait for me.” 我让他在外面等 means “I had him wait (in general),” which is odd unless context already makes clear whom/what he’s waiting for. Keep 等我 to specify the object.

How do I show that I already gave the instruction? Where does 了 go?

Use sentence-final 了 (often with 已经): 我已经让他在外面等我了。 This marks the instruction as completed. Avoid putting 了 directly after 让 here; 让了他在外面等我 would mean something else (“yielded to him”) and is not what you want.

How do I say “I told him not to wait for me outside”?

Use 别 before the verb you’re prohibiting: 我让他别在外面等我。 If you mean “I didn’t allow him to wait outside,” use 不让: 我不让他在外面等我。 The first is a directive; the second is about permission.

Is 在 here the same 在 used for the progressive “be doing”?

No. In 在外面, 在 is a preposition meaning “at/in.” The progressive uses 在 + verb, e.g., 他在等我 = “He is waiting for me,” which is a different structure.

Are 外面, 外边, and 外头 different?

All mean “outside.” 外面 is the most neutral; 外边/外头 are common too with mild regional flavor. Any of them works: 我让他在外面/外边/外头等我.

Any tone or pronunciation tips for this sentence?

让 ràng: fourth (falling) tone.
外面 wàimian: wài is fourth tone; mian is often a light (neutral) tone in fast speech.
等 děng: third tone.