Learn Hardcore Chinese: Nǐ jīntiān děi kàn wán zhè běn shū ma? - Do you have to finish reading this book today?

得 (děi): have to/must (colloquial, often due to external requirement or schedule). Example: 我得走了 (I have to go).
要: need to/want to/be going to. In 你今天要看完…吗？, it can mean plan or requirement; context decides.
必须: must (strong, formal or emphatic). 你今天必须把这本书看完吗？ sounds quite firm.
需要: need to (neutral, matter-of-fact). 你今天需要看完…吗？ is gentler/polite.
应该: should/ought to (recommendation, not obligation). 你今天应该看完…吗？ asks about what’s advisable, not required.

In everyday speech, 得/要/需要 are the common choices for this kind of question; 必须 is stronger/formal; 应该 softens to a suggestion.

How do I pronounce this 得, and how do I tell it from the other three “de’s” (的 / 得 / 地)?

得 (děi) (3rd tone): modal meaning must/have to. Example: 你得去.
的 (de) (neutral tone): attributive/possessive. Example: 我的书.
地 (de) (neutral tone): links adverbial to verb. Example: 认真地看 (read carefully).
得 (de) (neutral tone after verbs): complement of degree/potential. Example: 看得很快 (read quickly).

Position helps: modal 得 (děi) comes before the verb (你得看…), while complement 得 (de) follows the verb (看得…).

Why use 看完 instead of just 看?

完 is a resultative complement meaning “to completion.”

看 = to read/look (the action).
看完 = to read something to the end; finish reading it.

So 得看完 means “must finish reading,” not merely “must read (some of it).”

What’s the difference between 看完 and 读完?

看完: most common for finishing a book, article, or anything you look at/read. Also used for movies/shows (看完电影).
读完: emphasizes the act of reading/studying a written text; sounds a bit more formal/academic. You can say 读完这本书, but not for movies.

Both are fine with books; 看完 is more general/colloquial.

Where does the object go with a resultative complement? Is 看这本书完 okay?

Two natural patterns:

V + 完 + object: 看完这本书 (used in your sentence).
把 + object + V + 完: 把这本书看完 (also very natural).

Do not say 看这本书完 (wrong order). You can also topicalize: 这本书看完了吗？

Can/should I add 了 after 完 in this sentence?

Not here. Your sentence is about a requirement today (future/ongoing), so no 了.
Use 了 when stating a completed fact:

我看完了这本书。
我把这本书看完了。

Is 吗 the only way to ask this yes–no question? What are alternatives?

Other natural patterns:

A-not-A: 你今天要不要把这本书看完？
是不是: 你今天是不是得看完这本书？ (seeking confirmation)
需不需要: 你今天需不需要看完这本书？

Rising intonation alone (without 吗 or an A-not-A form) is not standard in Mandarin.

How do I answer this question naturally?

Affirmative: 得。/ 要。/ 是的，我今天得看完。/ 嗯，得。
Negative: 不用。/ 不需要。/ 不必。/ 今天不用看完。 You can add a reason: 老师没要求。/ 截止日期在明天。

How do I negate the idea of necessity correctly? Is 不得 the opposite of 得 here?

Use:

不用 / 不需要 / 不必 = don’t need to.
不能 = cannot (ability/permission).
不得 = must not/prohibited (formal/legal), e.g., 这里不得吸烟.

So the opposite of 得 (must) in everyday speech is 不用/不需要/不必, not 不得.

What does 你今天看得完这本书吗？ mean? Is that the same as the original sentence?

Different meaning:

你今天得看完…吗？ = Do you have to finish… today? (necessity)
你今天看得完…吗？ = Can you finish… today? (ability/feasibility, using the potential complement 得 after the verb)

The negative for ability is 看不完.

How do I say “I can’t finish it today”?

我今天看不完（这本书）。 To soften: 我今天可能看不完。
If it’s about not needing to: 我今天不用看完。

Is the placement of 今天 fixed? Can I say 今天你得…?

Both are fine:

你今天得看完… (neutral, very common)
今天你得看完… (slight emphasis on “today”)

Time words typically go before the verb, either right after the subject or at the very start.

Why is the measure word 本 used with 书? Can I use 个?

Books take 本: 一本书.
Other classifiers:

一册 (volume, formal/for multi-volume works)
一部 (for long works like novels/films; e.g., 一部小说) Do not use 一个书.

Does 看完 already imply finishing the whole book? Do I need 全/全部/整本?

Yes, 看完 implies finishing it entirely.
You can add emphasis: 把这本书全看完 / 把整本书看完, but it’s not required.

What’s the difference between 吗 and 嘛 at the end of a sentence?

吗: neutral yes–no question particle. Example: …吗？
嘛: modal particle for obviousness, explaining, or softening.
你今天得看完这本书嘛 sounds like “Come on, you have to finish it (obviously),” not a genuine yes–no question.

Could I replace 得 with 要 here? Any nuance change?

Yes: 你今天要看完这本书吗？ is natural.
Nuance: 要 can sound more like plan/intention or requirement (context decides), while 得 leans toward an external obligation (schedule, rule, teacher’s demand). Both are common.

What’s the difference between 看过 and 看完?

看过 = have read/seen before (experiential; doesn’t imply finishing). 我看过这本书 = I’ve read/seen it before (maybe not all).
看完 = finished reading. 我看完这本书了 = I finished it.