I've gone through the tutorial about "A/an and the" from English Grammar Today in which I guess I understand the rules.
Regarding singular countable nouns, the tutorial states
We only use a/an with singular countable nouns
We can use the with singular and plural countable nouns
The tutorial does explicitly claims something like
there has to be an article before singular countable nouns.
However, in practice, the statement seems to appear to be a rule. Is my understanding correct?
This is the precise version, which is waiting to be reviewed.
"Usually, there has to be a determiner before singular countable nouns that are not proper nouns"