If we rewrite the above sentence in a lets say dull way the sentence is as follows:
This is an interesting program.(Not the interesting program)
a/an + adjective + countable nouns
Nouns are used with indefinite article,
a/an, when they are modified by descriptive attributes which bring out a special aspect of a quality, state, etc. expressed by the noun.
Now the grammar rule you need to know about exclamatory sentence states that if the noun in your sentence is plural, the correct choice is "what," not "how." "What" is acceptable with singular nouns as well.
As above so below:
What an interesting program this is!
Notice that exclamatory sentences are used to add excitement to your story or prose and to show a vast range of emotions: love, anger, happiness, confusion, elation or any other typed of exuberant emotion.They are reserved for powerful feelings so you won’t find them used to express a matter-of –fact emotion or serenity, or a sense of calm. Instead they deliver a jolt of feeling, which is why they’re so common in everyday speech.
In other words, an exclamatory sentence is used to express a sudden emotion. It could be fear, anger, anxiety, admiration, excitement etc. So we are not talking about sentences that make a casual statement, ask a question, or give a command.
Here are some tips for constructing exclamatory sentences.
what + a/an before a singular noun.
What a surprise!
What an interesting program!
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.
Examples are given below
What a nice evening!
What a pleasant surprise!
What awful weather!
How noble of him to do that!
How clever of you!
Note that it is possible to rewrite many of these sentences using a structure with it.
It was very noble of him to do that.
It was very clever of him.
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.
Another way that can be used to pull the reader into your words is by inserting exclamatory words or interjections in a sentence:
Wow, that's an interesting program!
Brilliant, you solved the puzzle!