We use one or the other depending on the context. There isn't one simple rule differentiating them. We use ser in specific situations and estar in other specific situations. We have many specific lessons on these essential verbs. Here are some of the A1 and A2 lessons:
Using ser (not estar) to describe relationships
Using ser (not estar) to express an essential or permanent characteristic
Using ser (not estar) to express origin or nationality
Using estar (not ser) when talking about locations
Using estar (not ser) when talking about physical condition of something, someone or a place
I hope this helps. There are more lessons on ser and estar for more advance levels.
I use the following to try and help myself remember.
Ser is Permanent such as occupation DOCTOR
Origin (where you come from)
And Estar for temporary such as a location or PLACE
Condition (Such as tired)
Emotion (such as happy)
But I am no expert.
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