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 "tense" 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:
What is the difference between them? Is there semantic difference? Can you give exaple when 是...的 can be used, but 了 can't?