”Ser” is the verb ”To be” indicating a permanent state or one that can be changed, but which is characteristic of the person or object by nature.
”Estar” is the verb ”To be”, but in the sense that something or someone is temporarily in a condition.
Examples: ”Ela é triste.” / ”She is sad.”
It means that the person is sad by nature, she is not sad just for a moment, it is part of their personality.
”Ela está triste.” / ”She is sad.”
It means that the person is sad for a moment because of some situation that happened and that left her that way and not because sadness is part of her personality, her characteristic.
”Ele é rico.”/ ”He is rich.”
It means that wealth is part of his life and we don’t know if one day he will stop being rich.
”Ele está rico.” / ”He is rich.”
It gives the idea that he is rich in a temporary state, but that at some point he may cease to be rich. It can also convey the idea that he was not rich and had a lot of money recently for some reason, in this case he asks for a context.
Example: ”Ele era muito pobre, mas ganhou na loteria e agora está rico.”
”He was very poor, but he won the lottery and is now rich.”
”Ser” is a permanent quality, or condition, a fact. As for ”estar”, think of it using the same root for the English word "state" (as in "a sate of being") - something is (or is found at) a certain state, at a certain time. For example: "O céu é azul" - ”the sky is blue”. Well, in general, with good weather, the sky is blue, right? A "normal" sky, shall we say, IS blue, i.e, as a rule, it’s blue. However, we know there's cloudy skies, too. So, you could say ”O céu está nublado” (the sky is cloudy). It’s cloudy now, due to bad weather - the sky is in a state of being cloudy. "Um círculo é redondo" (A circle is round - a circle is not temporarily round - it is always round. Roundness defines a circle, it IS round. It's not round NOW, and then later it will have corners). Note that ”estar” also relates to places (the state of being in a place): ”Eu estou na biblioteca” (I’m at the library - i.e., I am now, currently, in this physical state/position of being at a library). And, of course, by the same logic, it's also used for physical states, including your own: ”Eu estou com febre” (I have a temperature/I have a fever - I’m in a febrile state. You wouldn’t say ”I AM a fever”, would you? By the same token, one could say: "Eu estou com o braço direito levantado" (I have my right arm raised" - because that's the physical state I'm in, with my arm raised. BTW, in colloquial spoken Brazilian Portugues, "estou" is shortened to "tô" (that's a long, grave "o", like when you say "Oh, I get it now") HTH.
LuisTostaOctober 2023The colloquial ”tou” is also used in Portugal. Actually, it is quite common (colloquially) to drop the initial ”es” in many conjugated forms of the verb ”estar” (’tive, ’tiveram, ’tava, ’taremos. ...)
”ser” is used to express inherent qualities and characteristics, while ”estar” is used to express temporary conditions and states.