Is “permanent versus temporary” a good rule?

No. It is the rule most learners are given and it fails on the commonest cases. Madrid’s location is permanent and takes estar; being young is temporary and takes ser. Identity versus condition predicts correctly where permanence does not.

Which adjectives change meaning?

About a dozen carry real weight: aburrido (boring/bored), listo (clever/ready), rico (wealthy/delicious), verde (green/unripe), vivo (sharp/alive), malo (bad/ill), bueno (good/tasty), orgulloso (arrogant/proud), seguro (safe/sure), despierto (bright/awake). All follow the pattern: ser is the trait, estar is the state.

Does French have the same problem?

No, French uses être for both, which is why French speakers find this hard in the opposite direction from English speakers. Portuguese and Italian split it much as Spanish does, so if you have one of those the distinction transfers almost intact.

Which one to useA2

Ser vs estar

The short answer

Ser says what something is: identity, origin, profession, material, time. Estar says how something is right now, and where it is: condition, mood, location, and anything in progress. Es aburrido means he is boring; está aburrido means he is bored.

At a glance

Ser or Estar?

Compare	SerWhat it is. Identity and definition.	EstarHow and where it is. Condition and place.
What it expresses	Identity, essence, definition	State, condition, position
Location	Only for events: la fiesta es en mi casa	For everything else: el libro está en la mesa
Time and dates	Son las tres, hoy es martes	Not used
Feelings and health	Not used	Estoy cansado, está enfermo
With a past participle	Passive of action: fue construido	Resulting state: está construido
Continuous tenses	Not used	Estoy trabajando

How to choose

Ask these, in order

1
Are you saying what something or someone is?
Ser. Nationality, profession, relationship, material, possession, definition.
Es arquitecta y es de Bilbao.
She’s an architect and she’s from Bilbao.
2
Are you saying where something is?
Estar, for everything except events. A building, a person, a country all take estar.
Madrid está en el centro del país.
Madrid is in the centre of the country.
NBPermanent location still takes estar. Only events take ser: el concierto es en el parque.
3
Are you describing a condition that could change?
Estar. Tired, ill, angry, open, broken, ready, busy.
La tienda está cerrada los domingos.
The shop is closed on Sundays.
4
Is the adjective one that changes meaning?
Choose by which meaning you want. About a dozen adjectives work with both.
Es listo. / Está listo.
He’s clever. / He’s ready.

Side by side

Each one, in full

Ser

When to use ser

What it is. Identity and definition.

Ser defines. It covers identity and profession, origin and nationality, material and possession, time and dates, and impersonal statements like es importante or es verdad.

The reliable frame is that ser answers the question “what is it?”. If you could replace the sentence with a definition, ser is the verb, even for things that will change, like age or a job.

El anillo es de plata.

The ring is silver.

Son las nueve y media.

It’s half past nine.

La reunión es en la sala grande.

The meeting is in the big room.

NBAn event, so ser, the one place ser handles location.

Estar

When to use estar

How and where it is. Condition and place.

Estar describes. It covers physical location, temporary conditions, moods and health, and the result of a change: la puerta está abierta, estoy cansado, está roto.

It is also the auxiliary for continuous tenses: estoy trabajando, and it appears in a set of fixed expressions worth learning whole: estar de acuerdo, estar a punto de, estar de pie, estar de vacaciones.

¿Dónde estás? Estoy en la estación.

Where are you? I’m at the station.

Estoy de acuerdo contigo.

I agree with you.

Está lloviendo desde ayer.

It’s been raining since yesterday.

NBContinuous tenses always take estar.

Mistakes to avoid

What learners actually get wrong

Madrid es en España.
Madrid está en España.
Location takes estar, however permanent it is. Learners taught ser as “permanent” and estar as “temporary” get this backwards, which is why the identity-versus-condition framing is the more reliable one.
Soy cansado.
Estoy cansado.
Tiredness is a condition, so it takes estar. Ser cansado does exist and means tiresome, a description of your personality rather than your afternoon.
La clase está aburrida.
La clase es aburrida.
If you mean the class itself is dull, it takes ser. Está aburrida says the class is feeling bored. The same flip applies to listo, rico, verde, vivo and malo, and picking the wrong one can be unintentionally rude.
Estoy profesor.
Soy profesor.
A profession is identity, so it takes ser, even for a job you will leave. Estar with a job exists in estar de profesor, which specifically means working as one temporarily.

Questions

Asked and answered

Is “permanent versus temporary” a good rule?: No. It is the rule most learners are given and it fails on the commonest cases. Madrid’s location is permanent and takes estar; being young is temporary and takes ser. Identity versus condition predicts correctly where permanence does not.
Which adjectives change meaning?: About a dozen carry real weight: aburrido (boring/bored), listo (clever/ready), rico (wealthy/delicious), verde (green/unripe), vivo (sharp/alive), malo (bad/ill), bueno (good/tasty), orgulloso (arrogant/proud), seguro (safe/sure), despierto (bright/awake). All follow the pattern: ser is the trait, estar is the state.
Does French have the same problem?: No, French uses être for both, which is why French speakers find this hard in the opposite direction from English speakers. Portuguese and Italian split it much as Spanish does, so if you have one of those the distinction transfers almost intact.