Today, Spanish, Galician, Portuguese, Catalan, and (to a lesser extent) Italian commonly use two copulas, one from each of the Latin verbs.
Thus its inflectional paradigm is a combination: most of it derives from svm (to be) but the present subjunctive appears to come from sedeo (to sit) via the Old Spanish verb seer.
E.g. derivation from sedeo: Derivation from svm: The infinitive (on which the modern future and conditional are based) could have derived from either or both: In the early part of the second millennium, in texts such as the Cantar de Mio Cid, ser was still used mostly as in Latin, and there was little place for estar; sentences like Es pagado, e davos su amor, "He is satisfied, and he gives you his favour" are found, where modern Spanish might have Queda contento, or Está satisfecho, y le da su favor.
Indeed, ser is etymologically related to the English words "essence" and "is", and estar with "state", "status", "standing", "stance" and "stay".
The expression como una cabra (with the implied loco omitted) is used with estar to mean "mad as a hatter", "crazy as a loon".
For example, someone who had been depressed for a prolonged period, and then had a life changing experience like a new career or long-term relationship, might say ahora yo soy feliz, meaning, "now I am happy".
A special use of ser, which expresses neither a nature nor a state but an action, is the formation of the passive voice: Estar is usually used with adjectives that derive from past participles of verbs since the use of ser would sound like a verb in the passive voice.
Such adjectives in any case generally refer to states: A special example of this tendency is what happens with words indicating prohibition and suchlike.
To say the same thing with a past participle, estar (or quedar) is required, in order to differentiate it from the use of ser with a past participle implying an action expressed in the passive voice: This fine nuance is not encountered in other Romance languages, which do not go to such lengths to distinguish between passives and similar-sounding phrases.
The use of estar for location may be easier for English speakers to grasp if they recall that it is derived from Latin stare, "to stand."
For example, one might say to a taxi driver the following phrases, to indicate that one has arrived: The difference becomes clear if aquí is changed to esta calle: Es aquí and es esta calle express the idea that "this is the place", a concept quite different from what is expressed by estar.
This happiness often turns out to be a transitory state, a person may nevertheless declare yo soy feliz as a statement of optimism that goes beyond the description of today's mood that is expressed by any phrase with estar.
When not a state but a change of state is referred to, the expression is quedar contento or alegrarse: Ella quedó muy contenta cuando yo le dije que ella había ganado = "she was very glad when I told her that she had won"; Yo me alegro de que vosotros hayáis llegado = "I am glad that you (plural) have come".
In the excerpt from the Cantar de Mio Cid above, one can see that "to be happy" a thousand years ago was ser pagado (meaning "to be paid" in current Spanish).
As in Spanish, estar derived from Latin sto / stare: The copula ser developed both from svm and sedeo.
Thus its inflectional paradigm is a combination of these two Latin verbs: most tenses derive from svm and a few from sedeo.
The main difference between Spanish and Portuguese lies in the interpretation of the concept of state versus essence and in the generalisations in some constructions.
For example, unlike Spanish, Portuguese does not require estar with past participles; in this case, it follows the general rule regarding state/essence.
The Italian copulas did not undergo the same development as in other languages, having preserved the Vulgar Latin forms essere and stare.
Occitan has just one copula, estre; which is also written èsser, ester, estar in diverse dialects.
The latter meant to ‘stand’, ‘stay’ or ‘stop’, and might have been used as a copula in a similar way to other Romance languages.
With phonetic evolution, the forms of each verb tended to be confused with one another, with the result that estre finally absorbed ester; around the same time, most words beginning with est- changed to ét- or êt-.
The only clear traces of ester (or éter if we bear in mind the loss of the s) in the modern copula are the past participle and the imperfect.
This is a highly defective verb and mostly exists in the infinitive, although present and past participle are infrequently used as well.
However, in the Vulgar Latin dialects spoken in modern-day Romania, its evolved form fiere replaced essere, maybe due to the similarity between the stems fi- of fieri and fu- of esse (which is not coincidental, as both stems descend from the Proto-Indo-European verb *bʰuH-).