1

As far as I know 了 has two main functions. 1st is the completion marker, and the 2nd one is change of state marker. Here are some examples in which I can't figure it out the function and necessity of 了.

  1. 办公室在三楼,我们上楼去, 先把借书证办了。

What's the difference between

先把借书证办了.

and

先把借书证办.

  1. 我真想都买了。

  2. A: 你要做什么? B: 当然是写作业了.

3 Answers 3

2

了 can also be a 尾語 (final particle) similar to 啊

When removing 了 from a sentence doesn't make the sentence invalid, that 了 is most likely not function as a 'complete marker' or 'change of state marker' but a final particle that serves to emphasize the sentence (also indicates the sentence has ended)

  • Remove 了 from 先把借书证办了 and write 先把借书证办 would make the sentence invalid. The function of 了 here is to indicate the completion of the verb 办

  • Remove 了 from 我真想都买了 and write 我真想都买 would change the meaning of the sentence. The function of 了 here is to indicate the completion of the verb 买

  • Remove 了 from 当然是写作业了 and write 当然是写作业 doesn't change the meaning of the sentence, the function of 了 here is as a final particle (indicates the sentence has ended and put emphasis on the sentence, like an exclamation

2

The following are my opinions:

  1. When a sentence ends up in a verb, it requires an auxiliary word/particle to indicate the completion of the action is someone's interest -
  • 先把借书证办了/好. - (It is in your interest), first to finish the application for a library card.

  • 我真想都买了/掉. - (It is in my interest), I really want to sell all.

Note that without the auxiliary word/particle 了/好/掉, the sentences are grammatically incorrect.

  1. When the sentence ends up in a verb phrase (V + Noun), 了 is a modal particle for setting the tone, and it is optional:
  • 当然是写作业了/啦/呀. = 当然是写作业.

Note that while both mean the same, the former answer has a casual tone, and is less serious about the task (写作业) than the latter.

1

Well, 了 can't be a “completion marker”, because nothing has been "completed". You must go upstairs and do something.

Also, nothing has “changed”, because you have not yet gone upstairs, so 了 can't be a "change of state marker“.

An intention has been stated, so look at 了 as emphasis.

You may replace 了 with 好, or with 好了。

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