When to use the verbs ser and estar often causes problems. Both verbs mean to be;

Ser is used for permanent characteristics eg the sky is blue, gold is  expensive, he is tall etc

Estar is used for temporary characteristics eg he's angry, I'm unwell, the drink is cold, the sky is grey today,  he's idiotic this afternoon etc

BUT remember the exception which breaks this rule ......

Estar is used to describe (geographical) position egMadrid is in the centre of Spain, he's in the corner etc.

Revise all of this in your grammar book.

Here's an exercise: