An exclamatory sentence is used express a sudden emotion. It could be fear, anger, anxiety, admiration, excitement etc.
Here are some tips for constructing exclamatory sentences.
Use what a before a singular noun.
• What a surprise!
Before an abstract noun or a plural noun, use what without a.
• What awful weather!
Use how before a modifying adjective, an adverb or a verb.
• How beautiful the flower is!
Examples are given below
What a nice evening!
What a pleasant surprise!
How clever of your brother!
Note: It is possible to rewrite many of these sentences using a structure with it.
• How kind of him to help the poor man!
It was kind of him to help the poor man!
• How thoughtless of him not to have locked the car!
It was thoughtless of him not to have locked the car!