Learn Hardcore Chinese: Tā yígòng huán le liǎng běn shū, míngtiān zài huán yì běn. - She returned two books in total; she’ll return one more tomorrow.

Here 还 is pronounced huán (second tone), meaning “to return (something borrowed).”
Don’t confuse it with 还 hái (“still; also”). With an object like 书 and the perfective 了, huán “to return” is the only reading that fits.
Examples:
- 我还了书。 Wǒ huán le shū. = I returned the book(s).
- 我还有书。 Wǒ hái yǒu shū. = I still have books. (different word/meaning)

What does 了 do after 还?

This 了 is the perfective marker: 还了 means the returning action was completed.
A common pattern is: Verb + 了 + number + measure word + noun to report a completed quantity, e.g., 还了两本书.
There’s no sentence-final 了 here because we’re simply reporting a completed action and then talking about a future action.

Why is 两 (liǎng) used instead of 二 (èr)?

Use 两 before a measure word to say “two of [something],” e.g., 两本书.
二 is for counting, numbers in addresses, ordinal numbers, math, etc. Examples:
- 两个人 two people, but 二楼 second floor, 第二本 the second book.

Do I need the measure word 本 with 书?

Yes. In Chinese, numerals require a measure word between the number and the noun.
For books and other bound volumes, the standard measure word is 本: 一本书, 两本书.
You cannot say just 两书.

Why is 一 pronounced yì in 一本, but yí in 一共?

Tone sandhi for 一:
- Before a 4th-tone syllable, 一 changes to 2nd tone: yí. Example: 一共 yígòng, 一定 yídìng.
- Before a 1st/2nd/3rd-tone syllable, 一 changes to 4th tone: yì. Example: 一本 yìběn, 一杯 yìbēi.
- When said alone (counting “one”), it’s yī.

Where can 一共 go in the sentence?

The most natural placement is before the verb phrase: 她一共还了两本书.
Don’t say 她还了一共两本书 (unnatural).
You can also add it as an afterthought when listing: 她还了书，一共两本 (informal).
For existence rather than an action, use 一共有: 一共有两本书 (“there are two books in total”).

Why is 再 used instead of 又?

再 signals a repeated or additional action in the future or in a hypothetical plan: 明天再还一本 = return one more tomorrow.
又 describes repetition for past or present situations: 她昨天又还了一本 = she returned yet another one yesterday.
Rough guide: future = 再, past = 又.

Does 一共 include the book to be returned tomorrow?

No. 一共还了两本书 refers to the completed returns up to now. The 再 clause adds a future, additional one.

Why is the subject missing in the second clause?

Chinese often drops a repeated subject when it’s clear from context. The second clause 明天再还一本 still refers to 她.

Can I add 会 to talk about the future, like 明天会再还一本?

Yes. 会 makes it sound like a prediction: 明天会再还一本 = She will return one more tomorrow.
Without 会, the time word 明天 is already enough to indicate a future plan.

Where should the time word 明天 go?

How would I say “return the books to the library”?

Add a recipient with 给 and optionally use 把:
- 把书还给图书馆。
- 她把那两本书还给了图书馆。
- With direction: 把书还回图书馆。

Why not use 过 instead of 了 after 还?

过 marks life experience (done it before, at least once) without tying it to a specific occasion or quantity.
她还过书 = She has returned books before (experience).
她还了两本书 = She returned two books (this specific completion). That’s what we need here.

Can I use the 把 construction here?

Yes, especially if the books are specific: 她一共把那两本书还了。 or 她一共把两本书还给了图书馆。
把 works best when the object is known/specific (e.g., 那两本书 “those two books”).

Why is 书 omitted after 一本 in the second clause?

When the noun has just been mentioned, Chinese often omits it to avoid repetition. 一本 here clearly means “one (book).”
Full form would be 明天再还一本书, but it’s natural to drop 书.

Can I drop 了 after 还 and say 她一共还两本书?

Without 了, it can sound like a general habit/plan (“she (in total) returns two books”) rather than a completed action.
To report what she already did, keep 了: 她一共还了两本书.

Is 总共 the same as 一共?

Yes, they’re near-synonyms meaning “in total/altogether.” 总共 can feel a bit more formal, but both are common: 她总共还了两本书.