Hello,
There are two grammar points about 是:
1, 是 Sentence, it is used to indicate judgment and indicating existence.
the formula: A (Noun) + 是/不是 + B (Noun)
https://www.cchatty.com/grammar/10401
今天是晴天。(OK)
2, 是……的, it is used to emphasize something.
the formula: Subject + 是 + Verb/Adjective + 的
https://www.cchatty.com/grammar/10381
她是很漂亮。(NOK)
她是很漂亮的。(OK)
(more)