Learn Hardcore Chinese: Tā zuìjìn yígòng jiè le sān běn shū, míngtiān jìde huán. - He has borrowed three books in total recently; remember to return them tomorrow.

The 了 here is the perfective aspect marker. It marks the action 借 (to borrow) as completed, not “past tense” in the English sense. So 借了三本书 means “borrowed three books (completed).” Chinese doesn’t mark tense the way English does; time words like 最近 (recently) provide the time frame. Without 了, the sentence would not clearly indicate a completed event.

Who is actually supposed to return the books in 明天记得还? The sentence starts with 他…

With 记得 used as an imperative, the implied subject is usually “you” (你). So as written, it sounds like: “He recently borrowed three books; remember (you) to return them tomorrow,” which is a bit mismatched.

If you’re talking to the borrower: 你最近一共借了三本书，明天记得还。
If you want to keep it third-person: 他最近一共借了三本书，明天要还/得还。
If you’re asking someone to remind him: 明天记得让他还 or 记得提醒他明天还。

Why is there no object after 还? Should it be 明天记得还书?

Chinese often drops objects when they’re obvious from context. Here, 书 is understood from the first clause. You can say:

明天记得还书。
明天记得把书还了。 (emphasizes completing the return)
明天记得还给图书馆/老师。 (specifies the recipient) Using 它们 for “them” is possible but less natural than repeating 书.

How do I know 还 here is huán “to return” and not hái “still/also”?

Context and collocation. With 借 and with an implied object like 书, 还 (huán) “to return” is the only reading that fits. Also, 明天 (tomorrow) doesn’t make sense with 还 (hái) “still.” Common return collocations include 还书, 还给 (return to [someone]).

Where do 最近 and 一共 go? Could I say 他一共最近借了…?

Typical order is: Subject + Time + Adverbs + Verb + (了) + Quantity + Measure + Noun.

Natural: 他最近一共借了三本书。 or 最近他一共借了三本书。
Unnatural: 他一共最近借了… (the order “一共最近” sounds wrong) Put 最近 (time) before the verb; 一共 (total) also goes before the verb.

Why 三本书 and not 三个书?

Chinese requires a measure word between a number and a noun. For books, the standard measure is 本, so 三本书 is correct. 三个书 is wrong. Alternatives you may see:

三册书 (cè; formal or for volumes)
三部书 (bù; often for multi-volume works or films/operas)

Can I say 他最近一共借过三本书?

Using 过 marks past experience at an indefinite time. 一共 counts a definite total, so 一共…过 is atypical. 他最近借过三本书 can mean “he has (at some point recently) borrowed three books,” but it doesn’t present a clear, completed total the way 借了三本书 does. For a concrete tally in a time frame like 最近, use 了.

Could I put 了 at the end, like 他最近一共借三本书了?

The sentence-final 了 (change-of-state) can emphasize “by now it’s three.” The most natural emphatic form is to keep the verb 了 and add a sentence-final 了:

他最近一共借了三本书了。 (By now, the total has reached three.) Using only the sentence-final 了 without the verb 了 is less standard here.

What does 记得 do here? How is it different from 记住 or 别忘了?

记得 (jìde) = remember (to). Soft, everyday reminder: 明天记得还。
记住 (jìzhù) = remember/commit to memory; as an imperative it’s stronger: 记住明天要还！
别忘了 (bié wàng le) = don’t forget; also a common reminder: 明天别忘了还。 You can strengthen 记得 with 要: 明天要记得还。

Is 明天记得还 or 记得明天还 better?

Both are fine. Chinese typically places time expressions early, so 明天记得还 is very natural. 记得明天还 is also common, especially in speech.

Can I add 去 as in 明天记得去还?

Yes. 去 emphasizes the movement to a place to return the books (e.g., the library): 明天记得去还（书）。 It’s optional; use it when the “going” aspect is relevant.

Can 一共 be replaced by 总共?

Yes. 一共 and 总共 both mean “in total.” 一共 is slightly more colloquial; 总共 can feel a touch more formal, but in daily speech they’re interchangeable here.

Why is there just a comma between the two parts? Should it be two sentences?

Chinese often links related clauses with a comma. You could also write two sentences:

他最近一共借了三本书。明天记得还。 A conjunction isn’t necessary; the relationship (statement + reminder) is clear from context.

How do I state who borrowed from whom or who lent to whom with 借?

Borrow from: A 跟/向/从 B 借 C
Example: 他向图书馆借了三本书。
Lend to: A 把 C 借给 B or A 借 C 给 B
Example: 我把这本书借给了他。/ 我借这本书给他。 Pairing: 借 (borrow/lend) goes with 还 (return).

Any pronunciation tips for tricky words here?

一共 = yí gòng (tone sandhi: 一 changes to 2nd tone before a 4th-tone syllable)
记得 = jìde (the 得 here is neutral tone)
还 (return) = huán (2nd tone). Don’t confuse with hái (also/still).
了 (aspect) = le (neutral tone after the verb)
Full line: Tā zuìjìn yí gòng jiè le sān běn shū, míngtiān jìde huán.