了 vs 是...的 as "factuality markers"
(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.