The Spanish verb ‘to be’ has two conjugations: ‘ser’ meaning ‘to be mentally’ and ‘estar’, meaning ‘to be physically.’ In English, we find ourselves having to say “I am physically sick,” while in Spanish, such is not a problem. One simply uses ‘estar’ for the physical sickness, while using ‘ser’ for mental sickness. Note, however, that to be mentally ill is a physical ailment, hence the proper use is ‘estoy enfermo,’ not ‘soy un enfermo.’ The reason for this is that mental illness is an illness of the brain, not of the spiritual being of the individual being addressed. Hence, ‘you are a sick person,’ meaning your soul or the spiritual part of your mind is devious, is expressed with ‘ser:’ ‘eres un enfermo.’ ‘Ser’ therefore, is a qualitative state of meaning, while ‘estar’ is a quantitative one.
A better way to see the difference between ‘ser’ y ‘estar’ is by looking at the origin of the word. ‘Estar’ is a mix of two Latin words, the noun ‘ego’ (I) and the verb ‘stō’ (stand), to which regular endings of the Present stem have been added. Thus ego+ stō+y was rendered e+sto+y or ‘estoy.’ The Latin verb indicates physical presence; the individual is standing there, physically. Thus, ‘estoy malo’ means ‘I am physically ill.’ ‘Ser’ on the other hand, has a reflexive connotation. In the Latin, ‘his own’ is expressed as ‘suus’ but the soul, ‘anima’ is defined both in plural and feminine connotations. Thus the proper Reflexive Possessive is ‘suae.’ ‘Soy’ thus shows that it is not the physical being of the individual who is sick, but rather that part of the soul that is not linked to the body at all, his being; a thing of the soul herself ‘suae’ or of the soul itself ‘sua.’ ‘Soy malo’ thus indicates a state of being bad of spirit, while ‘Estoy malo’ indicates badness of physicality.
Shakespeare would have had a trip with this, I know it.