What is the subject in the sentence suddenly, i got it! ?

"Suddenly, I got it!"

the subject is "I." the verb "got" applies to "I," which makes it the subject of the sentence because "I" is the one "getting" something, but furthermore the sentence is in subject-object-verb form: I (subject) got (verb) it (object). 

According to Mark Twain, "standards" are the lasting ideas people have, whereas "fashions" are just temporary trends or beliefs that change across the years.

As an example of "standards," Twain cites Shakespeare because, for centuries, his pieces have been adopted and recreated by people worldwide.

As an example of "fashions," Twain considers the different styles of music, writing, and technology being the latter the most changing with the passing of time.