how

adverb: in what way or manner; by what means
- How do you solve this math problem?
adverb: to what degree, extent, or amount
- How tall is that building?
adverb: in what state or condition
- How is your grandmother feeling today?
adverb: for what reason; why
- How is it that you arrived so early?
adverb: used to ask about the quality or nature of an experience
- How was your vacation in Italy?
conjunction: introducing a clause that explains the manner, reason, or circumstances
- I couldn't understand how the accident happened.
interjection: exclamation expressing surprise, admiration, or disbelief
- How! I never thought you'd arrive so early.