Lesson 5.4 - why 'le' for this sentence

In lesson 5.4, this sentence appears: 对不起,明天我不能陪你了。 Duìbùqǐ, míngtiān wǒ bù néng péi nǐ le. I am sorry, but I can't spend time with you tomorrow. Why does the sentence end with 了 (le)? This is not a completed action nor is it a change in state. Those are two cases where 了 (le) is needed. I know there are other cases where one uses 了 (le) but I have not found them described in my Chinese grammar book.

I did a Google search using "uses of le chinese" and found "Chinese Grammar Wiki" that explains other uses of " le". I hope this helps. http://resources.allsetlearning.com/chinese/grammar/Uses_of_%22le%22

That is a useful resource. I read through it and still cannot find how it applies to the sentence that I quoted. What would the sentence mean without the 了 (le)? The only thing I can think of is that maybe this indicates a change in expectations. Dave expected Lin to spend tomorrow with him and Lin is gently telling him that she is not available. Does the 了 (le) particle soften the sentence?

My wife who speaks Chinese says that "le" can be omitted or left in. Leaving it in makes the sentence more friendly so that you don't offend people. It does in effect soften the sentence to make it more polite.

大家好！ In order to understand why the 了 is added we must look further back in the conversation where we find out that Lin has already accompanied Dave on a trip to the Forbidden City. When Dave asks her what she is doing tomorrow and she says she is busy, the 了 is referring to the change that she can no longer or not keep on showing him around as she has her own plans. 的 would also work and add a polite tone to the refusal. 了 is a difficult piece of grammar to get your head around, mainly because its use may not be so clear cut or as in this case, be referring to something a few lines back in the conversation. I hope this helps and keep up the good work! 加油！ - Lin Ping

Grammar of 了。 了1 ：completion I. S+V+了+Number+Measure word+Obj 我们开了五门课。 II. S+V1+了+O1+V2+O2 我吃了晚饭去图书馆看书。 了2 ：change or a new situation happedned. N/V+了: 今天星期五了 or 他包了 Adj.+了 : 叶子红了。 V+Result Complement+了 ： 作业写完了吗？ 写完了。 In this sentence is 了2. Just before I can go, but now I cannot go.