Is this true that one shouldn't use indefinite articles, 'a' and 'an', with abstract nouns because they aren't countable? But definite article, 'the', could be used with any kind of a noun. Please let me know. Thanks.
I think you can sometimes use uncountable abstract nouns with the indefinite article, when you mean to say "a kind of".

Example taken from the COCA (Corpus of Contemporary American English):
Betsy Bennett's delicately colored paintings are remarkable for the way they reveal a beauty that is otherwise concealed from the eye in her ordinary, workaday subjects.

(= a kind of beauty)

You might also be interested in this paragraph:
In a way it`s true.
  • We can use a\an with singular countable nouns: a beach, a student...
  • But there are many cases when the same noun can be both: countable and uncountable with a difference in meaning.
E.g.: There is a spare room.(count) - You can`t sit here. There isn`t room.(uncount).
  • We don`t use the when we are thinking of smth as a general idea. For instance we use such words as: prison, hospital, school, university, church.
- without the, when we are thinking of what these places are used for(its general idea)

E.g.: He is in prison for robbery.
- with the, when we are thinking of these places as particular buildings
E.g.: The workmen went to the prison to repair the roof.
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Thanks, Der, and welcome to the forums.
 Kooyeen's reply was promoted to an answer.