Learn Hardcore Chinese: wǒ kàn le nà běn shū.

Here 了 after the verb 看 is the perfective aspect marker. It signals that the action of reading happened and reached completion as an event. It does not itself mark “past tense” in the English sense; Chinese is tenseless, and time is inferred from context or time words.

Can I drop 了 and still be correct?

If you say 我看那本书, without context it tends to sound habitual (“I read that book [as a rule]”) or like a plan (“I’ll read that book”), not a completed one-time event.
For a specific, completed action (what English would often put in simple past), it’s more natural to include 了: 我看了那本书.
With a clear time word like 昨天, you still normally keep 了: 我昨天看了那本书.

How do I negate this sentence?

Use 没(有) and drop 了:

我没(有)看那本书。 = I didn’t read that book. Don’t use 不 with the perfective. 不 is for habitual, generic, or planned actions (e.g., 我不看那本书 = I don’t/won’t read that book).

Does 我看了那本书 mean I finished the whole book?

Often it’s understood as “read it (likely finished),” but 了 itself doesn’t guarantee “to the end.” To be explicit, use a result complement:

我把那本书看完了。 (I finished reading that book.)
If you only read part of it, say: 我看了那本书的一部分。

What’s the difference between 了 and 过 (e.g., 我看过那本书)?

了 (perfective) = a specific, completed event (often tied to a particular time or occasion).
过 (experiential) = you have the experience of doing it at least once, with no focus on when or completion on a specific occasion. So 我看了那本书 ≈ I (did) read that book (this time). 我看过那本书 ≈ I have read that book before (at some point).

Why do we need 本 before 书?

Chinese requires a measure word (classifier) with demonstratives and numbers. 本 is the classifier for bound volumes like books: 那本书 = that book. You generally cannot say 那书. Other examples: 一份报纸 (a newspaper), 一张纸 (a sheet of paper), 一部电影 (a movie).

What’s the difference between 那本书 and 那一本书?

那本书 = that book (neutral).
那一本书 adds contrast/emphasis on “one” (that one specific book, as opposed to others).

Could I say 我读了那本书 instead of 我看了那本书?

Yes, but nuance differs:

看 is broader (“look/read/watch”). 看书 commonly means “to read (a book).”
读 focuses specifically on reading aloud or reading text; it can sound a bit more formal/literary in some contexts. Both are fine here, with 看 more colloquial.

Where do I put time words like “yesterday”?

Time expressions typically go before the verb:

我昨天看了那本书。 You can also front the time as topic:
昨天我看了那本书。

What about sentence-final 了? Is 我看那本书了 okay, and how is it different?

Sentence-final 了 marks a new situation/change-of-state. 我看那本书了 can mean “I’m (now) going to read that book” or “I (have now) read that book (as opposed to before).” It’s about a state change rather than simply marking a completed event. Your sentence with verb-了 (我看了那本书) is the straightforward “I read that book (completed action).”

Can I use both 了s (e.g., 我看了那本书了)?

In this simple statement, no—don’t use both. Double 了 appears in other patterns (e.g., duration-in-progress: 他病了三天了), but not here.

How do I say “I was reading that book (at that time)”?

Use the progressive with 在 (or 正在):

我在看那本书。
With a past time: 我昨天在看那本书。

How do I say “I’ve already read that book”?

Add 已经:

我已经看了那本书。
Or with a stronger completion result: 我已经把那本书看完了。

How do I say “I read that book twice”?

Use the frequency complement:

我看了那本书两遍。 You can also topicalize: 那本书我看了两遍。

Can I use the 把 construction here?

Yes, to foreground the object and the result:

我把那本书看了。 (neutral completion)
More informative with a result complement: 我把那本书看完了。 (finished reading)

Can I drop the subject 我?

Yes, if context makes it clear. For example, in answer to “What did you do?”, you could say:

看了那本书。 Chinese often omits subjects when they’re understood.

Any pronunciation tips for these words?

我 wǒ (3rd tone)
看 kàn (4th tone)
了 le (neutral tone here)
那 nà (4th tone; often pronounced nèi in colloquial speech before classifiers)
本 běn (3rd tone)
书 shū (1st tone)

Is 看 also “to watch”? When should I not use it?

看 means “look/read/watch.” You 看书 (read books), 看电影 (watch a movie), 看手机 (look at a phone). You wouldn’t use 读 for watching movies; 读 is for reading text.

Can I write spaces between the characters like in the prompt?

Standard Chinese writing does not use spaces between characters. You would normally write: 我看了那本书。

How would I ask “Which book did you read?” and avoid mixing up 那 and 哪?

Use 哪 (nǎ) for “which”:

你看了哪本书？ Don’t confuse it with 那 (nà) “that.”