Why is he not saying "a cardboard" instead of just "cardboard"?
"Cardboard" is used here without an article because it is referring to multiple pieces of cardboard, which is more general than just one piece of cardboard. However, if you wanted to refer to a piece of "cardboard" specifically, you would use the definite article "the" in front of cardboard. "A cardboard" is not grammatically correct because "cardboard" is an uncountable noun, and so the plural of "cardboard" is still "cardboard".
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