How lucky you are!
How quickly the time passed!
How can also modify a verb.
How we laughed!
After what there can be a noun phrase with a/an or without an article.
What a journey we had!
What idiots we've been!
The noun phrase often has an adjective.
What a stupid mistake you made!
What lovely flowers these are!
An exclamation can also be just a phrase with how or what.
How lucky!
What a journey! What lovely flowers!
2 Other exclamations
Any phrase or short sentence can be an exclamation.
Oh no!
Lovely!
You idiot!
Stop!
Look out!
Oh, my God!
There is usually a greater rise or fall of the voice than in other types of sentences. In writing we use an exclamation mark (!).
Some exclamations have the form of a negative question. The voice rises then falls.
Aren’t you lucky! (= How lucky you are!)
Didn't we laugh! (= How we laughed!)