Ser = To be
Estar = To be in a state of (easy to remember e**sta**r, **sta**te
“Ser” is used for when you are describing long-term or even permanent characteristics of a noun (object, person or place) or verb. Also think of “ser” as when you’re describing **facts** about something, or a characteristic that isn’t changed by its surroundings. The fact that it’s Thursday may be temporary, but it’s a fact that is true regardless. Nothing can change that it’s Thursday; the day of the week isn’t a state of being.
Ella es inteligente
Esta clase es muy aburrida
Creo que correr es bastante divertido
Las mesas son rojas
None of these are “states” that the objects or verbs are in.
You’re young, but you’re not young “for now”, it’s going to be a very long time before you’re old. It would be slightly odd in English to say “You’re young… for now”.
She is intelligent, but it’s a part of her as a person. She’s not just intelligent “for the time being”.
This class is boring, but it’s not just in a state of being boring. I phrase it as a fact because that’s just how I think it is. This class is very boring.
I think that running is quite fun. Not just some runs that I go on, but all runs at all times. I think it’s a fun activity in general. Again, since it’s my opinion, I state it as an unchanging fact. Value your opinions!
The tables are red. They’re not just red “at this moment”, since I don’t have any plans to paint them. You’re describing a fact about the tables
“Ser” is also used before other nouns. This is more to point out _what_ an object or person is, not necessarily describe it.
Soy un médico
Ella es una fanática de Britney Spears
Sure, I could quite my job as a doctor, but we’re describing _what_ I am. Same applies for the Britney Spears fan.
“Estar” is for a state something is in. It can be real and factual, but it’s not necessarily a fact about the object in particular. If you isolated that object from its environment, would it still be in that state? The state an object is in is caused by its surroundings.
La montaña está a la derecha del lago
Mi madre esta orgullosa de mi
Estamos jugando un videojuego
I’m sad, but it’s just a state I’m in. I’m not necessarily a “sad” person in general.
The mountain is to the right of the lake, but it’s not a fact about the mountain itself. It’s a fact about how the mountain relates to its environment and the objects around it. Remove the lake, and the statement is no longer true.
My mum is proud of me probably because of something I’ve done. She’s not necessarily a proud person in general.
We’re playing a videogame right now. It’s a state we’re all in.
Swapping “ser” for “estar” can completely change the meaning for these reasons:
“Estoy molesto” means “I’m annoyed” or “I’m bothered”
But if we swap that for “Soy molesto”, it means “I am **annoying**” or “I am **bothersome**”. It now describes a fact about what I’m like as a person, instead of a state of emotion.
“Estoy molesto porque mi hermano es molesto” literally means “I’m annoyed because my brother is annoying”.
Please tell me if I got anything wrong.