Hola !
"The apple is green" ( the apple isn't ripe )= "La manzana ESTÁ verde" = condition
"The apple is green" ( the color of the apple is green ) = "La manzana ES verde" = essence
In English, the verb "to be" can be used to tell how something is (the condition) and what something is (the essence).
How is the apple? = Cómo está la manzana ?
It is unripe. = Está verde ( No está madura )
What color is the apple? = De qué color es la manzana ?
It is green. = Es verde ( La manzana es de color verde )
In Spanish, a different verb is used to express "to be" depending on whether the speaker intends to address a condition or an essential quality.
To address condition, use ESTAR (it's an irregular verb )
If you are talking about how something is, use ESTAR = to indicate a state or condition, often one resulting from a change: "Estoy muy triste" = I am very sad.
To indicate location: "Estamos en casa" = We are at home.
Preceding DE to form various idioms of condition or state: "Estamos de viaje" = We are on a trip.
With a present participle to form a progressive tense: "Él está cantando" = He is singing.
If you are talking about what something is, use SER.
To indicate existence, and with adjectives to indicate inherent, innate or essential characteristics: " La casa es grande" = The house is big.
To indicate origin, nature or identity: "Soy de Argentina" = I'm from Argentina.
To indicate possession or ownership, either literal or figurative: "Es mi casa" = It is my house.
With past participles to form passive voice.
To tell time and to indicate where an event takes place.
Also in impersonal statements: "Es importante" = It is important.
If you need more information about the use of SER and ESTAR, please tell me, I can send to you a few grammar texts about this topic.
Best wishes
Sofía