Exclamatives introduce (or start) exclamative clause and exclamative sentence, both of which express strong emotions and opinions. The former is a noun clause; the latter an independent clause. There are two exclamatives: what and how.
Exclamative determiner
Of the two exclamatives, what is a determiner (predeterminer, to be specific) in the clause it introduces and hence is called exclamative determiner. It is followed by indefinite article a or an. Examples:
What a wonderful idea just struck me! [Independent clause. What is a predeterminer and is followed by a.]
What a journey we experienced last summer! [Independent clause]
He couldn’t believe what a wonderful idea had just struck him. [Dependent clause]
It’s hard to believe what a journey we experienced last summer. [Dependent clause]
Exclamative adverb
How is an adverb in the clause it introduces and hence is called exclamative adverb. It’s an intensifying adverb, which pre-modifies adjective, adverb, or even a clause. Examples:
How incredible their performance was! [Independent clause. How pre-modifies adjective incredible.]
How quickly the weather changed! [Independent clause. How pre-modifies adverb quickly.]
He mentioned how incredible their performance was. [Dependent clause]
She was amazed at how quickly the weather changed. [Dependent clause]