This is a good news.
This is good news.
Which one is correct? Clarify the difference
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"news" is singular, uncountable. So you can't say "a news," instead you would say "This is good news" just as in "This is good water."
News itself is uncountable. Uncountable nouns don't take indefinite articles a and an; at least not directly. Let me explain.
So far, we understood that
This is *a good news.
is ungrammatical, while the latter form isn't. What if you do need to specify a one about news? That's where classifiers1 come into play.
A classifier, sometimes called a counter word, is a word or morpheme that is used to accompany nouns and can be considered to "classify" the noun depending on the type of its referent. - Wikipedia
The classifier for news, jewelry, meat, cake2 etc. is piece:
Sam just heard this shocking piece of news about the epidemic from the bus radio.
Pfft, this job is a piece of cake for a tough guy like you!
Thus, if you want to make your first sentence grammatical, use piece along with the indefinite article a:
This is a good piece of news.
1: Warning: Do not mix classifiers with noun classes. Inhaling the mixture might result in body inflammation and is highly toxic.
2: Also, a slice of cake.