Adjectives need no là: Chị ấy đẹp

In English an adjective needs "is", "am" or "are": "I am tired." In Vietnamese the adjective does that job by itself: Tôi mệt. So mệt means both "tired" and "be tired", and a person or thing plus an adjective is already a full sentence: Phòng này rộng. This room is big.

Because it works like a verb, an adjective takes the same words a verb does. Không in front makes it negative: Món này không cay. Rất before it, or lắm after it, makes it stronger: rất cay, cay lắm. Có … không asks about it, though the có is usually dropped: Sài Gòn nóng không? Rồi goes with it as with a verb: Em đói rồi. I'm hungry now. Đang works only for a passing state: Em đang bận, I'm busy right now, but not Phòng này đang rộng.

Là says what someone or something is, so a noun follows it: Anh Minh là kỹ sư. It doesn't go before a plain adjective, and neither does không phải, which also belongs with nouns. The negative of rộng is simply không rộng.

Right after a noun, an adjective usually describes it: phòng rộng is "a big room". Once the noun is closed off, for example with này (this), the adjective becomes the statement: Phòng này rộng.

You may meet là before an adjective in formal writing or for emphasis. That's a separate, later pattern.