The verbs "ser" (to be) and "estar" (to be) are two of the most important verbs in Spanish, which can be used to express who you are, how you feel, or how something is or isn't. They are irregular verbs, so abide by different conjugation rules than regular verbs.
Yo soy mujer.
I am a woman.
La chica no es rubia.
The girl is not blond.
Tú estás embarazada.
You are pregnant.
Él no está enfadado.
He is not angry.
How to Conjugate "Ser":
We can see that the conjugation is unique and does not resemble the regular Spanish verb conjugation (e.g. ar conjugation: -o, -as, -a, -amos, -áis, an). Thus, you have to memorize the forms of "ser".
How to Conjugate "Estar":
In the case of "estar", the first person singular is irregular (estoy), but you can follow the -ar conjugation rules ((-o), -as, -a, -amos, -áis, -an) for the conjugation of the other persons.
Rule 1: Since the verb "estar" is stressed on the last syllable, instead of the penultimate -as usual, it carries a tilde (accent) in every conjugation except the first person singular and plural (estoy/estamos).
Negation with "ser" and "estar"
Rule 2: To express the negative of a sentence, add a "no" (not) before the verb "ser" or "estar". Here are the examples:
El perro no es blanco. El perro es negro.
The dog is not white. The dog is black.
Yo no estoy ocupada, estoy disponible.
I am not occupied, I am disponible.
How to use the verbs "ser" and "estar"
Rule 3: In English, the verb "to be" expresses the meaning of both "ser" and "estar". In Spanish, however, one must distinguish which verb to use according to context.
The verb "ser" is used in invariant contexts, such as descriptions:
Yo soy alemana.
I am German.
Ellos son hermanos.
They are brother and sister.
If one wants to talk about temporary states, such as whereabouts or states of mind, Spanish uses the verb "estar:"
Yo estoy triste.
I am sad.
Ella está en la plaza del mercado.
She is at the marketplace.
Delimitation of verbs
Rule 4: Some adjectives can be used with both verbs, but the meaning would be different. The following examples will clarify this:
Yo estoy feliz.
I am happy. (Right now)
Yo soy feliz.
I am happy. (A happy person in general)
While the sentence with "estar" expresses a temporary state, such as "Today I am happy," the sentence with "ser" describes a permanent quality in the sense of "I am a happy/cheerful person."
Ser or estar? - "She is tired."