Learn Hardcore Chinese: wǒ yǐwéi tā bù lái le, dànshì tā zhōngyú dào le. - I thought she wasn’t coming, but she finally arrived.

以为 means “to assume (wrongly).” It almost always implies the belief turned out to be incorrect, as shown by the second clause here.
想 is “to think/plan” (neutral; no implication of being wrong).
觉得 is “to feel/think (opinion or impression).”
认为 is “to believe/hold the view that,” often more formal and not necessarily wrong. Example: 我以为她不来了 implies you were mistaken; 我想她不来 is a neutral guess; 我觉得她不来 is your feeling; 我认为她不来 is a considered view.

Why is it 不 in 她不来了, not 没?

没(有)来 = “didn’t come” (a past fact).
不来 = “won’t come / refuses to come” (non-past, volitional or scheduled).
不来了 adds the idea of “not coming anymore” (a change of plan/state). Here you were expecting her to come, then believed she wouldn’t (prospective), so 不来了 fits better than the factual past 没来.

Why are there two 了 in one sentence? Is that okay?

Yes. They’re different 了’s:

她不来了 uses sentence-final/modal 了 (change-of-state): “(she’s) not coming anymore.”
她终于到了 uses aspect 了 after the verb to mark completion: “(she) arrived.” They can co-exist because they serve different functions.

What’s the nuance difference between 不来 and 不来了?

不来: neutral negation about (present/future) coming; refusal or plan.
不来了: “not coming anymore,” highlighting a change relative to prior expectation or arrangement. It often carries a slight sense of disappointment or update.

How is 不来了 different from 来不了?

不来了: not coming anymore (choice/decision/change of plan).
来不了: cannot come (inability/constraint), e.g., traffic jam, illness. Context: “She changed her mind” → 不来了; “She’s stuck and can’t make it” → 来不了.

Why 到了 and not 来了? Can I say 她终于来了?

Both are possible, but the focus shifts:

到了 emphasizes the result “arrived (at the destination).”
来了 emphasizes motion towards the speaker’s location. Use it when she arrived where the speaker is. If you’re at the destination and she reached you, 她终于来了 is fine. If you’re emphasizing arrival as an endpoint (possibly with an implied place), 她终于到了 is natural.

Does 到 need a place after it? Why is there no place here?

到 often takes a destination: 到了公司/机场/这儿. But if the destination is obvious from context, you can omit it and just say 到了 (“arrived”). That’s what’s happening here.

What does 终于 convey, and where do I put it?

终于 means “at long last; eventually,” with a sense of relief or culmination after waiting/effort.
It typically goes before the verb/adjective: 她终于到了, 他终于同意了.
It can be used with negatives when the awaited result is negative: 她终于没来 (“in the end she didn’t come,” often with resignation).

Can I use 最后 instead of 终于?

They’re different:

终于 = culmination of a long-awaited result (speaker’s emotion/relief).
最后 = the final point in a sequence (“in the end/at last”), neutral about emotion; can simply mean “the last step.” In this sentence, 终于 is more natural because it conveys relief that she arrived. 最后她到了 is possible but feels more like sequencing.

Is 但是 the only “but” I can use? How do 可是/不过/但 differ?

All can show contrast:

但是/但: more formal, slightly stronger contrast. 但 is shorter and more written.
可是: more colloquial/spoken.
不过: “however/only that,” softer contrast or a mild qualification. All would work here with small tone differences.

Can I drop the second 她 and just say 但是终于到了?

In casual speech, if the subject is crystal-clear, you can omit it. But generally, repeating 她 keeps the sentence clear and smooth. 但是终于到了 can sound abrupt or ambiguous in writing.

Can I say 我以为她不会来了? How does that differ from 不来了?

Yes. 不会来了 adds 会 to make it explicitly predictive: “I thought she wouldn’t be coming (anymore).” It can sound a bit more tentative or forecast-like. 不来了 is a straightforward change-of-plan statement you believed to be true.

What about 我还以为她不来了? What does 还 do?

还以为 is an idiomatic way to underscore a mistaken assumption, similar to “I actually thought / I was under the impression.” It adds a mild “as it turns out, I was wrong” flavor. It fits this context well.

Could I say 我以为她没来 instead? Would the rest still work?

我以为她没来 = “I thought she hadn’t come (already),” a belief about a completed past fact. It contrasts more naturally with something like 可她其实已经来了 (“but she had already arrived”), not with 但是她终于到了 (which narrates a later eventual arrival). So it changes the timeline and doesn’t match the original continuation.

Why not just say 她终于来 without 了?

With punctual, completed events like “arrive/come,” Chinese typically marks completion/result (e.g., 来了/到了) or uses a complement. 她终于来 sounds incomplete unless further modified (e.g., in certain stylized or exclamatory contexts like 她终于来啦). In standard narration, prefer 她终于来了/到了.