I read the grammatical rule that we use "what " where we have non countable things and "what a " where we can count things.
But at the same time we say
"what a nonsense !" How can we use "what a " here . we cant count nonsense. Similarly in many sentences where I think we should not use "what a" we still use it.
Is there something wrong with my interpretation of nonsense as a non countable thing?
Who says that, elcid?-- the normal phrase is 'what nonsense!' And 'nonsense' is indeed uncountable. However, I would be interested in other cases you have found.
Rather than the countable / uncountable paradigm, can we not abide by singular / plural? "Nonsense" in that context is a plural noun (collectively referring to things of no sense), so it wouldn't be proper to use "a". You can see it works if you observe case:

- What a great idea!
- What great ideas!
- What fine roses you have in your garden!
- What a wonderful thing to say!
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It doesn't work that way, Moby. If 'nonsense' is a plural noun, what is the singular form? Can we have 'two nonsense'? None of your examples show anything more than 'what a + singular countable common noun' vs 'what + plural countable common noun'.
Maybe I should have avoided the word plural. I don't know how to describe it the textbook way, but nouns that refer to a collective such as "nonsense", "wood", "knowledge", etc., should not be preceded by "a". Ignore the examples I gave, but the point is that "What a nonsense" is not correct because "nonsense" is certainly not a singular noun, whatever else it may be.
Hi, Mr., well, I'm not sure about these:
What a shame
What a mess
Is there something like: what shames, what messes.
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I have a feeling that some of these are becoming idiomatic, Latin; we don't see the plural often; but yes, shame, mess and pity can be countable nouns. I've never heard 'what shames' or 'what pities', but 'what messes children make' I should think is reasonably common!
Actually, "nonsense" (as well as "wood", "knowledge", etc.) are all singular noncount nouns.

Maybe the question should not be when to use "what" vs. "what a". Maybe it's just a matter of saying that, in this type of expression, "what" precedes whatever the usual indefinite noun phrase normally is.

noncount nouns, singular: no article, noun only ("nonsense", "butter", "faith", etc.)
count nouns, singular: "a" + noun ("a singer", "a muffin", "a shoe", "a suggestion", etc.)
count nouns, plural: no article, noun only ("singers", "shoes", "boulders", etc.)


What total nonsense! What a great suggestion! What beautiful shoes!

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