When is 'prejudice' used as a countable noun, or an uncountable noun? - - At your discretion and depending on context. -- Only the grammar structure varies, as it does between all countable vs non-count nouns.
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Then you might try googling the two terms 'countable nouns' and 'uncountable/non-count nouns'. There are many web pages with explanations at all levels. You can start here: