Ser vs estar: when to use each

English uses one verb, "to be", for almost everything. Spanish splits the job between two verbs. Once you see the pattern, most of the choices become predictable.

The short version: ser describes what something is (its identity, nature, origin or lasting qualities). Estar describes where something is, how it is right now, or the result of a change.

Use ser for

Identity and description of permanent traits: Soy profesora. (I am a teacher.) Ella es alta. (She is tall.)
Origin and nationality: Somos de Finlandia. (We are from Finland.)
Time, dates and events: Son las tres. (It is three o'clock.) La fiesta es mañana. (The party is tomorrow.)
Material and ownership: La mesa es de madera. (The table is wooden.) El coche es mío. (The car is mine.)
Characteristics that define a thing: Ser inteligente describes a person's nature.

Use estar for

Location: Estoy en casa. (I am at home.) Madrid está en España. (Madrid is in Spain.)
Temporary states and feelings: Estoy cansado. (I am tired.) Estamos contentos. (We are happy.)
Results and conditions: La puerta está abierta. (The door is open.)
The progressive tense: Estoy estudiando. (I am studying.) This is estar plus the gerund (-ando, -iendo).

The same adjective, two meanings

Some adjectives change meaning with the verb. These are the ones to learn first:

Ser	Estar
Es aburrido. He is boring.	Está aburrido. He is bored.
Es listo. He is clever.	Está listo. He is ready.
Es malo. He is bad (nature).	Está malo. He is ill.
Es rico. He is wealthy.	Está rico. It tastes good.

Soy aburrido would tell people you are a dull person. Estoy aburrido means you are bored right now. The verb carries the meaning.

A quick test

Ask yourself: is this permanent, or is it a current state or a location?

If it is permanent, defining, or about time, origin or material, use ser.
If it is a place, a feeling, a condition or a temporary result, use estar.

Sometimes both are possible, and the meaning shifts slightly. Es muy nervioso describes someone with a nervous personality. Está muy nervioso describes someone who is nervous at this moment.

Common mistakes

Using ser for location. Soy en la biblioteca is wrong. Say Estoy en la biblioteca.
Using estar for origin. Estoy de Londres is wrong. Say Soy de Londres.
Using ser for feelings. Soy feliz is correct for a person who is generally happy. For today's mood, say Estoy feliz.
Forgetting that the verb changes the meaning of adjectives. Check whether the sentence describes the thing itself or its current state.

Practice

Choose ser or estar for each sentence. Ask yourself whether the sentence is permanent or temporary, then pick.

Next, read Preterite vs imperfect to see how the past tenses handle the same question of permanent versus temporary.