Somewhere, someone got the idea that estar is used with adjectives to show something is temporary and that ser is used for permanent things. These are inaccurate ways of looking at the problem and dangerous ones to put in a student’s mind. It’s easy to prove with two sentences: JFK está muerto. Mi hermana es soltera.
The key to understanding when to use estar with adjectives is when the description is actually registering a change of condition, state or status.
In the first example above, JFK was alive, and then he wasn’t. In the second example, my sister was born single and still is; it is normal and expected to be born single and remain so, until someone comes along, Cupid fires his arrow and changes everything. When she gets married, I will say Mi hermana está casada to show the change in her civil status.
When the verb ser is used with adjectives, it denotes something inherent or natural about what is described – which is not the same as claiming the quality is permanent. For instance, I can say Juan está casado – which shows his legal status before society. But I can also say Juan es casado to show his attitude about the relationship, or my perception about how he seems to regard his marriage – that he is a married kind of guy. Yet, as most of us know, hopeful as he may be, even faithful as he might be, he still could wind up in divorce court.
As many textbooks note, some adjectives change their meaning depending on whether they are used with ser or estar. One frequent citation is to note that María está cansada means that she is tired, but María es cansada means she is boring.
Examine these two meanings, keeping in mind that estar is used to register, or recognize a change in her condition. She was not tired a while ago and now, the speaker notices that she is. As for es cansada, an assertion is being made about her personality, a quality the speaker views as normative or characteristic of María. That does not mean she is doomed to be forever boring or that being boring is a permanent characteristic – it merely means that the speaker views her as one that is best generally described as boring.
On a brighter note, happiness is normative but being in a good mood is fleeting. Hence, the adjective contento is used with estar but feliz is more often associated (among careful speakers) with happiness, something deeper than a momentary laugh (again, not necessarily permanent) and is thus used with ser.
Finally, estar often is used to register a surprise. Your aunt may not have seen you for a few years and upon noticing how you’ve grown, she may exclaim ¡Qué grande estás! even though, obviously, this is neither temporary or a sudden change. Tomorrow, she’ll be used to your height and describe you by saying Mi sobrino es alto.
By keeping this perspective, as you work with students as they learn the use of ser and estar beyond the fundamentals of health and location (estar) and identification, origin, profession, composition and ownership (ser), you will be able to help them avoid some entrenched fallacies and prevent fossilizing some errors often associated with these two verbs.