Learn Hardcore Chinese: wǒ zài wàimiàn děng le hěn jiǔ.

Location use: 我在外面等了很久。
Progressive “be doing” is expressed with 正在 before the verb: 我正在外面等。
You don’t say 我在外面在等. If you want both location and progressive, use 我正在外面等 (or put 正在 after the location: 我在外面正在等).

Why is 了 after 等? Does it mean past tense?

This 了 is perfective aspect, marking the action as bounded/complete, not a “past tense.” 等了很久 = the waiting lasted a long time (as a completed stretch).

Completed duration: 等了很久
Habitual/future prediction: 要/会等很久 (no 了)
Negative for past/completed: 没等很久 (use 没; don’t use 不 here, and don’t put 了 after 没).

What changes if I add a sentence-final 了?

我在外面等了很久。 The waiting is viewed as completed (you may not be waiting anymore).
我在外面等了很久了。 The waiting has already lasted a long time up to now (you’re likely still waiting).
This is the “V + 了 + duration + 了” pattern for an ongoing situation. You can also say 我在外面等很久了 (the first 了 can be omitted).

Can I put 很久 somewhere else? Is 等很久 okay?

Duration normally follows the verb (often after 了): 等了很久.

For an ongoing situation now: 等很久了 (duration before the sentence-final 了).
Bare 等很久 is fine with modals or in non-completed contexts (e.g., 要等很久), but for a completed past stretch you typically say 等了很久.

Why use 很 before 久? Can I just say 久?

In modern speech, 久 is usually modified by a degree word; 很久 sounds natural and neutral. Bare 久 after a verb is rare/very formal. Common alternatives:

好久 (colloquial, a bit more emotive)
很长时间 (more literal)
半天 (colloquial “a long while,” not literally half a day)

What’s the difference between 很久 and 好久?

Both mean “a long time.” 好久 is slightly more colloquial/emphatic (think “such a long time”), and appears in set phrases like 好久不见. In this sentence either 等了很久 or 等了好久 works.

外面 vs 外边 vs 外头 — any difference? And what about the tone of 面?

外面, 外边, 外头 all mean “outside.” 外面/外边 are interchangeable; 外头 is a bit more colloquial/regional.
Pronunciation: the second syllable often takes a light (neutral) tone in speech: wàimian. Dictionaries may list wàimiàn as well; both are heard.

Can I drop 在 and say 我外面等了很久?

No; you normally need 在 to mark the location: 我在外面等了很久.
You can topicalize the place with a pause/comma: 在外面，我等了很久。

Where do I put the object I’m waiting for (e.g., “you,” “the bus”)?

Several natural options:

我在外面等你等了很久。 (V + O + V + 了 + duration)
我在外面等了你很久。 (also common in speech)
Avoid 我在外面等很久你 (wrong). If the object is the topic: 你，我在外面等了很久。

Can I use 正在 or 着 here?

Progressive: 我正在外面等。
Don’t combine 正在 with the perfective 了 on the same verb: 正在…等了… is ungrammatical.
等着 marks an ongoing state of waiting: 我在外面等着呢。 When specifying duration, prefer 等了X时间 rather than 等着X时间.

Any pronunciation/tone-sandhi tips?

很久 is 3–3, so the first becomes a rising tone: pronounced hén jiǔ.
了 is neutral tone (light and short).
A natural reading is: wǒ zài wàimian děng le hén jiǔ.

How do I negate or ask a yes–no question with this?

Yes–no question: 你在外面等了很久吗？
Negative (completed stretch didn’t happen/wasn’t long): 我没等很久。
For predictions or necessity, use 不/不用: 不用等很久, 不会等很久.

How do I say a specific amount of time, like “two hours”?

Put the duration after the verb (often after 了):

我在外面等了两个小时。
Ongoing up to now: 我在外面等了两个小时了。

Can I just say 我等了?

That’s usually incomplete. With perfective 了, Chinese prefers a complement (duration, result, object, etc.):

我等了很久。
我等到了他。 (result: “managed to wait until he arrived”)

Where do I put a time word like “yesterday”?

Canonical order is Time > Place > Action:

我昨天在外面等了很久。
Other time words follow the same rule: 我刚才/那天在外面等了很久。

How do I say “only a short while” or “not long”?

“Only a short while”: 我只等了一会儿。
“Not long” (past): 我没等多久。/ 我没等很久。
“Won’t take long” (prediction): 不会/不用等很久。