Why words such as , trouble, biology, flour, sleep,)..." doesn...

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Feb 28, 2019 02:33
Why words such as , trouble, biology, flour, sleep,)..." doesn't take the article a? although in some examples such as a biology class, there is an "a" letter?
what is the rule beyond that>?
Also, why "an advice when it comes a lone is a mistake, meaning that we need to write it advice with no article preceded ?