When should 之 be used as a particle? There doesn't seem to be a straightforward answer on Nciku. On Wiktionary, the entry states: Indicates that the previous word has possession of the next one. ...
I'm having a hard time understanding when to use 了 as a modal particle (instead of as an aspect particle). For example, what is the difference between the following two sentences. 现在几点？ 现在几点了？ ...