Cantonese adjectives: no 係 before a quality
In Cantonese an adjective already does the work of is. You do not put 係 in front of it. A plain statement almost always takes a degree word first, most often 好, which here means very or quite rather than good.
The adjective is the verb
English needs is hot, is quiet, is tall. Cantonese lets the adjective be the predicate, so 熱 already means is hot. In a full sentence you nearly always put a degree word in front, because a bare adjective can sound unfinished as a statement.
係 is for names, not qualities
佢係護士 says what they are. 佢好高 says how they are. Crossing the two gives 佢係高, which is the English is pushing its way in. Mandarin behaves the same way Cantonese does here, putting 很 where Cantonese puts 好 and leaving 是 out of a plain description, so this is not a habit carried over from Mandarin. Keep 係 for identity. See the identity guide.
As an adjective, 好 hou2 means good. As a degree word in front of another adjective, it means very or quite. 今日好熱 is not today is good hot. It is today is very hot. The degree page covers 好, 太, 幾, and 咁 as a set.
The 係...嘅 exception is a different pattern
You will hear 係 with an adjective inside the focus frame, as in 佢係好靚嘅. That is not a simple description. It is the 係...嘅 construction putting a quality in the spotlight, and it needs both halves of the frame. Learn the plain pattern first, then see the focus guide.
Common mistakes
Putting 係 in front of the adjective
今日係熱 and 佢係高 are the sentences English speakers reach for, because English cannot leave the is out. Spoken Cantonese wants 今日好熱 and 佢好高.
Leaving the adjective bare in a statement
今日熱 is understandable, but it often sounds clipped. 今日好熱 is the ordinary way to say it. Save the bare adjective for answers, notes, and contrast, where the degree is already obvious.
