In what nouns should I write the article "a"? Do I should to write article "a" for so words as "joy", "owner", "first aid", or just "aid", "endurance", "sandbox"?
You can use the article "a" only with singular countable nouns, not with uncountable nouns. "joy" and "aid" can be countable or uncountable, depending on context and sense in which the word is used "owner" and "sandbox" are countable "first aid" is uncountable "endurance" is normally uncountable, but it is not impossible for it to be countable A singular countable noun needs an article or other determiner (excepting special idiomatic patterns and expressions). However, this is of course not necessarily "a".
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You can use the article "a" only with singular countable nouns, not with uncountable nouns.
"joy" and "aid" can be countable or uncountable, depending on context and sense in which the word is used
"owner" and "sandbox" are countable
"first aid" is uncountable
"endurance" is normally uncountable, but it is not impossible for it to be countable
A singular countable noun