[unhammer]

You don't necessarily need knowledge to distinguish a topic from a subject though. It's a grammatical distinction. See

http://en.wikipedia.org/wiki/Topic%E2%80%93comment#Definitio... e.g. in the sentence (3) As for the little girl, the dog bit her "the dog" is the subject NP, "the little girl" is the topic. That toy example is perfectly parsable without semantics or even probabilities (though take any real-world sentence and I'm betting you'll need more than just grammar).