Exclamations with what
what a/an + adjective + singular countable noun
What a kind man! (NOT What kind man!)
What a beautiful flower! (NOT What beautiful flower!)
What a pleasant surprise! (NOT What pleasant surprise!)

what + adjective + uncountable / plural noun
What pleasant weather! (NOT What a pleasant weather!)
What beautiful flowers! (NOT What a beautiful flowers!)

What + object + subject + verb
What a beautiful smile you have!

Exclamations with howExclamations with how are now considered a little formal or old-fashioned. In this construction how is followed by an adjective.
            Blueberries! How nice!
The structure ‘how + adjective/adverb + subject + verb’ is also possible.
How beautiful she is! (NOT How she is beautiful!)
How beautifully she sings! (NOT How she sings beautifully!)
The structure ‘how + subject + verb’ is also possible.
How you have changed!

Such a/an + adjective + singular countable nounSuch a great experience! (NOT Such great experience!)
Such + adjective + uncountable / plural noun
They are such kind people! (NOT They are so kind people!)