了 vs 是...的 as "factuality markers" I'm a proponent of 了 as 'factuality marker' (that is it has nothing to do with 'perfectivity or completness of action'. See post: How exactly do time and &quot;tense&quot; work in Mandarin REALLY? for details about this theory.

Now, being what I'm, I've noticed second 'factuality marker' which acts almost exactly as 了. That marker is 是 in the 是...的 structure. There are couple of moments I would like to stress before the question:


Contrary to implicit 了, 是 explicitly (by nature of literal translation) states it's nature as "factuality marker".
As with 了, sentences with 是...的 tend to be translated in past tense.


The question: If 是...的 and 了 both render situation to facts, which in turn has correlation with being "in the past", what is the difference between them? Why have 2 'factualiy markers' (past tense markers by correlation).

Let's look at two senteces:



  我们是今天早上打网球的


  我们今天早上打网球了



What is the difference between them? Is there semantic difference? Can you give exaple when 是...的 can be used, but 了 can't?
(今天早上) (打网球)[了]

&quot;打网球&quot; is the fact; &quot;今天早上&quot; is the relative phrase that indicate what time it happened. Remove the relative phrase &quot;今天早上&quot; and write &quot;我们打网球了&quot;, the sentence is still a complete sentence. (we still know you've played tennis, just don't know when)

[是] ([今天早上打网球) [的]

the entire phrase &quot;今天早上打网球&quot; is the fact
If you remove the relative phrase &quot;今天早上&quot; and write &quot;我们是打网球的&quot;, the meaning would change completely. &quot;我们是打网球的&quot; means &quot;we are people who play tennis&quot;
The 'verb phrase' in  [是]+ (verb phrase )+[的] structure is a single item that cannot be broke apart
~

The (verb phrase)+ [了] structure can be in any tense
[是]+ (verb phrase )+[的] structure can only be in past tense

Example:

他(明天)要去美國(了)

&quot;去美國&quot; is the fact; 明天 is the relative phrase that indicate what time it will happen. Remove the relative phrase &quot;明天&quot; and write &quot;他要去美國了&quot;, the sentence is still a complete sentence.  (we still know you will go to American, just don't know when)

他(是)明天要去美國(的)

It is grammatically incorrect, because 要去美國 is not a face yet, not until tomorrow.

他(原定)(是) 明天要去美國(的)

It is grammatically correct, because 原定(originally planed) is a fact, and it happened in the past.