why

Từ điển American Heritage Dictionary 4th

why (hwī, wī)adv.
For what purpose, reason, or cause; with what intention, justification, or motive:
Why is the door shut? Why do birds sing?conj.
1. The reason, cause, or purpose for which:
I know why you left.
2. Usage Problem. On account of which; for which: “The reason why [regular verbs] are called regular is that we can predict what all the other three forms are” (Randolph Quirk). n. pl. whys
1. The cause or intention underlying a given action or situation:
studying the whys of antisocial behavior.
2. A difficult problem or question.interj.
Used to express mild surprise, indignation, or impatience. [Middle English, from Old English hwȳ. See kʷo- in Indo-European Roots.] 
Usage Note: Many critics have held that why is redundant in the expression the reason why, as in The reason why he accepted the nomination is not clear. While it is true that why could be eliminated from such examples with no loss to the sense, the construction has been used by reputable English writers since the Renaissance. See Usage Note at where.