One of the functions of 了 is marking the completion of an action. (了1) Another function is marking a change of state. (了2) Personally it is sometimes difficult for me to distinguish between these two functions.

Here are two examples, where 了 is used. But I can't figure it out which function it has. (completion of an action 了1, or change of state 了2)


1: I must return the book and complete this action.

2: (I didn't have to return the book before but) Now I have to return the book.


And if 了 acts as the completion marker, can it be used with present and future tenses?

The two examples provided above, seem to in present and future tense.

3 Answers 3


The sentences are from New Practical Chinese Reader 2. In the first case, the original snippet is:


image from textbook

(Note it's 还 = "return", not 换 = "exchange".)

You can see from the image that he hasn't yet returned the book, so it's definitely not a completion 了. It looks like a change of state 了: originally he said 我们就去借书 = "we're are going to borrow books", but realized he has to return his book first.

(On the other hand, it could just be a 语气词 (modal particle), because 把上次借的书还 without the 了 sounds unnatural or even wrong.)


image from textbook

Again, we can see from the image in the textbook that they haven't gone upstairs yet, so it cannot be a completion 了. I think it's a change of state 了; basically they were doing something (checking out the Beijing library), and now he's saying they need to do something else (go upstairs and get a library card).

I suggest avoiding relating 了 and "tense" (the keyword here is "aspect", not "tense").

I'm leaving. [present tense]

Wait until I get home, and then we can speak. [future tense]

I didn't go to the park yesterday. [past tense without 了]

In regards to...

And if 了 acts as the completion marker, can it be used with present and future tenses?

I think it would be fairly rare, if not impossible, to have something considered "completed" when we're talking about the present or future. It sounds like it'd just be a grammar error (outside of science fiction with time travel).

Also completion 了 is not used with all verbs. My teacher told me a good example of her student getting it wrong once: if I remember correctly her grandfather had passed away, and her student asked her:

What did he like to do?

The other thing that students don't realize (at least, it took me a long time to realize) is that, a lot of the time 了 is not used:

Yesterday, I saw a deer.

Yesterday, I studied hard.

This is unlike e.g. past tense, where it's far less optional.


Let's try to view 了1 and 了2from another angle:

(了2) indicates "change of state", which simply means "something wasn't before, now is".

  • 下雨 。(It wasn't raining, but now it is.)

  • 宝宝 会 说话 。(He couldn't before)

  • 他 当 经理 。(He wasn't a manager before.)

  • 我 饿 。(Now I am hungry.)

  • 妈妈 老 。(now mother is getting old.)

(了1) is called aspect, which is not the same as tense. Tense is about when an action happens: past, present, or future. With regards to 了 (le); aspect is about whether the action is complete in a certain time frame, regardless of tense.

"我得把书還[huán]了1" is an action to be complete in the future, "但是書外皮已經破了2 (it wasn't broken before), also note that 破 is not a verb representing action.

Hope this helps.





My understanding of this sentence is that to borrow books (去借书) is the main purpose to come to the library. (先把上次借的书还了) is a secondary task but it has to be done before 去借书. (maybe there is a limitation on the number of books that we can borrow). Therefore the final word 了 can be seen as a "modal particle", or just a convention to end this sentence.

You can also say 我先去还上次借的书.

Formally, this sentence can be reconstructed as 我先去还书, 再去借书.

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