How to determine if character can be used separetely?
Unlike alphabets in English, each Chinese character has its own meaning or meanings. A single character, usually cannot clearly express a specific meaning, therefore two characters would combine into a specific word, and two words may combine into one specific phase,

For example, 明 contains the following meanings:

[1] bright; light; brilliant 

[2] clear; understandable; [v] clarify; understand; obvious; evident 

[3] intelligent; clever 

[4] eyesight; seeing faculty

[5] day; daybreak; dawn 

[6] [v] state; show; assert 

[7] next (day or year)

[8] the Ming dynasty (1368-1644 A.D.) 

[9] a Chinese family name

With only one character, it is too general to know which meaning it is referred to, therefore we need to add a second character to form a specific term :

[1] 光(light) + 明(bright) = 光明(bright)

[2] 明(clear) + 白(plain) = 明白(understand)

[3] 聰(clever) + 明(intelligent) = 聰明(intelligent)

[4] 復(recover) + 明 (eyesight) = 復明(regain eyesight)

[5] 天(sky) + 明(daybreak)= 天明(daybreak) 

[6] 聲(voice) + 明(state)= 聲明(announce)

[7] 明(next) + 天(day) = 明天(tomorrow)

[8] 明(Ming) + 朝(dynasty)= 明朝(Ming dynasty)

[9] 明(Ming) + 先生(Mr.) = 明先生(Mr. Ming)

In some instances, the context would indicate which meaning of a character is referred, therefore using one character is possible; in other instances, a character only contains one single meaning, therefore is possible to be used alone.