I assume you have learned the standard rules as found on just about every grammar site on the Internet. Beyond that I'm not sure there's much help anyone can give (except to advise you in specific cases, of course). The unfortunate truth is that much of article use in English is idiomatic and must be learned by memorization and exposure to the language.
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SpacewaterFor example, The doorsill on a doorway. Why can't it be A doorsill on a doorway? or A doorsill on the doorway?Use "the" when you mean "one".