What is the difference in the meaning in the two following sentences?
a) "What a beautiful are your works!"
b) "How beautiful are your works!"
The context: I see my friend in his exposition of his paintings and I want to compliment him.
Exclamations are sentences spoken with emphasis. How and what are often used to form them. Compare these patterns :
How beautiful are your works? (A question )
How beautiful your works are! (An exclamation )
Another pattern uses what
What beautiful pictures (works of art)!
Or just Lovely! Aren't they beautiful!
Sentence (a) isn't grammatically correct because "beautiful" is not a noun. Compare that to "What a piece of work is man" or "What fools these mortals be." In this type of construction, "What a" must be followed by a singular noun with or without modifiers (or you can use "what" with no "a" followed by a plural noun). This is the adjective sense of "what". Here's the definition from Webster's Third New International Dictionary, Unabridged:
how remarkable for good or bad qualities : how surprising : how great : how small — used especially in exclamatory utterances and in dependent clauses of like nature
Sentence (b) is fine. It says that his works are beautiful, and beautiful is intensified by the use of the construction "how beautiful."
You could also say "Your paintings are beautiful" or "Your paintings are so beautiful." Those are simpler ways of saying the same thing.
What is the difference in the meaning in the two following sentences?
The words order isn't quite correct but both are saying the same thing.
If want to use the "what" pronoun for emphasis, as much as you could say:
"What big ears you have, Grandma," said Little Red Riding Hood.
"What a great dinner you've made, John!"
What a long trip!
What interesting news!
What interesting questions you've asked!
What little time she has she doesn't spend on shopping!
You could say:
What beautiful your works are!"
Same with the "how" adverb.
How difficult these questions are!
How small she is!
How beautifully he dances!
How big his ayes are!
I can't believe how fast they got here!
You could say:
How beautiful your works are!
That said, probably neither of those sentences sound quite natural for the context you present. Maybe something like:
Wow! Your work is really beautiful!
Note that there's no need for pluralising the work.