The definite article ה
One letter that changes the meaning
Hebrew has no indefinite article like English "a". It has only the definite ה, attached to the front of the word: סֵפֶר is a book, הַסֵּפֶר is the book we are talking about.
When a noun is definite, its adjective takes ה too. If the adjective does not, you have written a full sentence rather than a phrase.
Before א, ע and ר the article is pronounced longer: הָאִישׁ, הָעִיר, הָרֹאשׁ.
Phrase versus sentence
הַסֵּפֶר הַגָּדוֹל
ha-sefer ha-gadol
Phrase: the big book — two ה
הַסֵּפֶר גָּדוֹל.
ha-sefer gadol
Sentence: the book is big — one ה
The difference is a single letter, but it is the difference between describing something and making a statement about it.