Colloquial English is educated, but still not stuffy sounding. Colloquial is more formal than the way of speaking we would use to talk to our best friend, and doesn't include a lot of slang.
Formal English is both educated as well as stuck up. No one would use this form of speaking while talking, however it is often written. Despite being often written, this form of language is not as poular for witing as it was in the past. Formal English shall never contain slang.
Answer:
To express that the author's 'Muse' (or inspiration) is gone
Explanation:
"Where art thou, Muse, that thou forget'st so long
To speak of that which gives thee all thy might?
Spend'st thou thy fury on some worthless song,
Darkening thy power to lend base subjects light?"
I can sorta guess that this is asking something like "Where are you Muse, did you forget about me for so long?"
