What is the function of the 'O?
In this case, it is identifying that the following word is a proper noun. When it occurs as the first word of the sentence (instead of a verb occuring before it) it also indicates that the sentence is one of equivalence ("this = that").
What is the difference between kou and ko'u?
Isn't kou your and ko'u is my?
Does Hawaiian not translate the verb "to be"?
Hawaiian does not have a verb which is equivalent to "to be" and other structures are used to translate "to be".