What a pity! How awful!

A common way to form exclamations in English is to start with the words what or how. In Spanish we would say ¡qué...!, but in English you have to decide which of those two words to use.

How is followed by an adjective or an adverb:

How interesting! Let me have a look.
How lovely you look, darling!
How rude you are sometimes!
How beautifully she plays the piano!


What is followed by a noun. There may be (puede haber) other words, such as articles or adjectives, but there has to be (tiene que haber) a noun. You have to be careful with what exclamations because if the noun is singular and countable you need the article a(n):

What a pity!
What an interesting story!
What a lovely dress you're wearing!
What a rude man!
What a beautiful morning!

BUT in the story of Little Red Riding Hood (Caperucita Roja), you hear:

What big eyes you have!  What big ears you have! What big teeth you have! (a + plural)
Other examples:

What lovely hair she's got! (no nos referimos a UN pelo, sino a todo su cabello: uncountable)
What beautiful weather! (weather is uncountable)