Q. Why is "o" or "go" sometimes used before nouns?
A. "O （お）" or "go （ご）" is the respectful prefix (polite marker). It is used to express respect or simple politeness. Whether it shows respect or simple politeness, depends on the word. When it is used to express respect, it can't be used for yourself or family member.
|O-namae wa nan desu ka.
|What is you name?|
Watashi no namae wa Miki desu.
|My name is Miki.|
It is attached to a noun, an adjective or an adverb.
|adjective||O-genki desu ka.
|How are you?|
|Take your time.|
Generally speaking, "o" is used with native Japanese words and "go" with words of Chinese origin.
"O" or "go" isn't attached to all nouns. If you are a beginner, it might be better not to overuse them, though there are some nouns that sound harsh without them. "O-cha (Japanese tea)" and "o-kane (money)" are among them. Listen carefully to when and how native speakers use them.