Same poster as >>6:

I just found a bit more about "le" (sorry). It is generally used for (and I'm mostly quoting from my notes, for good or for ill) "le indicates an ongoing process, or something already done or completed". The only good example of this I have is "Shang ke le" [class has started] and "xia ke le" [class has ended]. They use the "le" particle to indicate that the stated condition has already occured - "class has already started", in a way.

I'm really not helping on this whole tl;dr; business. :P