As a general rule, the Real Academia indicates that the prefixes “are joined directly to the base word (antinatural, prerevolucionario, etc.)” However, in cases where the prefixes precede a word that is written with a capital letter (such as a name or an acronym), the correct thing is to use a hyphen. So we have examples like those offered by the RAE: anti-OTAN, anti-Mussolini. In addition, the hyphen is also useful to coordinate prefixes associated with the same base word, as in Se harán descuentos en casos de pre- o recontratación de servicios (Discounts will apply in cases of pre- or rehiring services).
In some cases, the prefixes are written separate from the lexical base, but the separation is only a space, no hyphen in between. This happens when the base of the prefix is made up of more than one word, as in these examples: el ex alto cargo del FMI (the ex high-ranking FMI official), los grupos anti pena de muerte (the anti death penalty groups). These prefixes are quite common and are known as separable prefixes.
Finally, a comment regarding the accent mark on prefixes: these are unstressed elements lacking prosodic accent, so they never have an accent mark (not even in those cases where they separated from their lexical base).
I hope this brief overview will help us remember the rules on the use of prefixes and save us the occasional need to consult a reference source!
To read the original Spanish post go to:
“El uso de los prefijos en español”