You use why in questions when you ask about the reasons for something. Why hasn't he brought the bill? Why didn't he stop me? Why can't I remember the exact year we married?
an introductory expression of surprise, disagreement, indignation, etc: why, don't be silly! Etymology: Old English hwī; related to Old Norse hvī, Gothic hveileiks what kind of, Latin quī
Why is an interrogative adverb that can be used alone and shows emotions like surprise or anger. For example: I went to Siberia.
