How and what are used in exclamations, how with an Adjective or Adverb alone, what with an Adjective and a Noun or a Noun alone, e.g.:
(Adjective)
How strange! (¡Qué extraño!)
(Adverb)
How fast they talk!
(¡Qué rápido que hablan!)
(Singular Countable Noun)
What a strange coincidence!
(¡Qué coincidencia tan rara!)
(Plural Countable Noun)
What wonderful flowers!
(¡Qué flores tan maravillosas!)
(Singular Countable Noun)
What an idiot!
(¡Qué imbécil!)
(Uncountable Noun)
What nonsense!
(¡Qué galimatías!)