Learn Hardcore Chinese: Cóng bā diǎn dào jiǔ diǎn wǒ zài túshūguǎn xuéxí. - From eight to nine, I study at the library.

从八点到九点，我在图书馆学习。 (Fronted time = stronger emphasis on the time range)
我从八点到九点在图书馆学习。 (Very common: Subject + time + place + verb)
我在图书馆从八点到九点学习。 (Also possible; slightly stronger emphasis on the place first)

All are grammatical; you choose based on what you want to highlight (time vs. place).

Is 从 required? Can I just say 八点到九点?

You can drop 从 in speech and writing: 八点到九点我在图书馆学习 is natural. Keeping 从 is also fine. For learners, two safe patterns are:

With 从: 从八点到九点我在图书馆学习。
Without 从: 八点到九点我在图书馆学习。

Do I have to repeat 点 in 从八点到九点?

Yes, repeat the measure word 点 after each number. Say 从八点到九点, not 从八到九点 (that sounds incomplete). You may add 钟 for the exact hour: 从八点钟到九点钟 (slightly more formal/explicit).

What’s the difference between 点 and 点钟?

点 = “o’clock” in everyday speech: 八点.
点钟 is a bit more formal/emphatic and is typically used only for exact hours: 八点钟. Don’t use 点钟 when adding minutes or “half”: say 八点半, 八点十五分, not 八点钟半.

Any tone-sandhi I should know here (e.g., 九点)?

Yes. 九点 is 3rd tone + 3rd tone. In speech, the first 3rd tone becomes a rising 2nd tone: pronounce it like jiú diǎn. Other words here don’t trigger special sandhi.

Is 在 here a location marker or a progressive marker (like “am -ing”)?

Here 在 introduces location: 在图书馆 = “at the library.” To mark the progressive “be doing,” you can put 在/正在 before the verb:

Location only: 我在图书馆学习。
Progressive (general): 我在学习。
Progressive at a place: 我正在图书馆学习。 (Clear and natural) Avoid doubling 在 (e.g., 我在图书馆在学习)—use 正在 instead.

How do I say this happened in the past? Do I need 了?

Chinese doesn’t require tense marking. A past time word is often enough:

昨天从八点到九点我在图书馆学习。 (already clearly past) You can add perfective 了 for a completed-event feel:
昨天从八点到九点我在图书馆学习了。 Both are acceptable; 了 highlights completion rather than “pastness.”

How do I say “for one hour” instead of “from 8 to 9”?

Use a duration phrase with 一个小时:

Neutral/habitual/scheduled: 我在图书馆学习一个小时。
Completed once: 我在图书馆学习了一个小时。 Note: With a duration, 了 after the verb commonly marks a completed event.

What’s the usual order of time and place phrases in Chinese?

A common template is: Subject + Time + Place + Verb (+ Object).

我（昨天）从八点到九点在图书馆学习。 You can front time or place for emphasis, but this base order is a safe default.

Should I add 里 after 图书馆 (i.e., 在图书馆里)?

在图书馆 is enough. 在图书馆里 adds “inside” and can stress being physically inside the building. Both are correct; use 里 when you want that nuance.

Can I put 在图书馆 after 学习 (e.g., 我学习在图书馆)?

No. Location phrases with 在 usually come before the verb phrase: 我在图书馆学习. Saying 我学习在图书馆 is ungrammatical in standard Mandarin.

How do I specify AM/PM (morning/evening)?

Use part-of-day words before the time:

Morning: 早上/上午八点
Afternoon: 下午两点
Evening: 晚上九点 For ranges: 从晚上八点到九点.

What’s the difference among 学习, 学, 看书, 读书, and 念书?

学习 (xuéxí): to study/learn (general, formal-neutral). Takes an object: 学习中文.
学 (xué): colloquial “to learn”: 学中文.
看书 (kànshū): to read books (activity of reading).
读书 (dúshū): to read/study; also “to attend school” in some contexts.
念书 (niànshū): “to study/attend school,” common in Taiwan. In your sentence, 学习 focuses on studying as an activity, not necessarily just reading.

Can I say this with a different pattern like “study until nine”?

Yes. Use 到 as “until” with the verb:

我在图书馆学到九点。 (studied/study until nine) If you also want the start time, add 从: 我从八点学到九点 (often understood to be at the same place if context is clear).

Is 至 interchangeable with 到?

至 is a more formal/literary counterpart to 到 in ranges:

从八点至九点 (formal, you’ll see it in notices/schedules) In everyday speech, 到 is more common.

Can I drop the subject 我?

Yes, if the subject is obvious from context (e.g., in a schedule or when answering a question):

从八点到九点在图书馆学习。 In a standalone sentence, keeping 我 is clearer.

What if I want to emphasize movement, like “go to the library to study”?

Use 去 for motion:

我从八点到九点去图书馆学习。 (focus on going there to study) Your original sentence 我在图书馆学习 emphasizes being at that location during that time.

Is a comma after the fronted time phrase required?

Optional. With a long fronted adverbial, a comma can improve readability:

从八点到九点，我在图书馆学习。 Without the comma is also fine in simple sentences.