In modern Spanish, the prefix desis one of the most productive prefixes, especially for neologisms. It means the opposite effect:
animar (to encourage) - desanimar (to discourage)
hacer (to do) - deshacer (to undo)
Its meaning is close the prefixes "dis-", "mis-" and "un-", compare:
acuerdo (agreement) - desacuerdo (disagreement)
confiar (confide) - desconfiar (mistrust)
tejer (to tie) - destejer (to untie)
montar (to assemble) - desmontar (to dismantle)
atornillar (to screw) - desatornillar (to unscrew)
Lately, this prefix has become so popular, that it started making lexical doublets:
aprender - desaprender (= olvidar)
andar - desandar (=volver)
liarme - desliarme (=liberarme)
It should be mentioned, though, these doublets are not always full synonyms: for example, desandar can be used with a direct object, like in
desandar el camino – to return the same way
The main semantic difference is that the verbs with the prefix des- indicate that the opposite action has been previously done: so, desaprender suggests something has already been studied, desliarse – somebody has been occupied, etc. This motivation seems to encourage creating such strange doublets like cambiar - descambiar (cambiar, devolver): in the verb descambiar, unlike cambiar, we can see the previous action comprar (naturally, it would be more logical to say descomprar, but it doesn´t exist).
It should be also mentioned that sometimes the subject of a new verb doesn’t coincide with the subject of the main verb:
mentir (to tell lies) – desmentir (to deny lies):
El presidente ha desmentido la información que ha dado El País
Here are some examples of the use of this prefix with a possible translation:
gravar (to tax) - desgravar (to eliminate tax)
autorizar (to authorize) - desautorizar (to deprive of authority)
acreditar (to accredit) - desacreditar (to discredit)
The prefix des- is also used with nouns, often deverbal:
amor – desamor (heartbreak)
hora – deshora (late or inappropriate hour)
Sometimes there is no direct deverbal noun, just the opposite!
alojar – desalojar
BUT: *
alojo– desalojo (evacuation)
The prefix des-, although very rarely, can be used with adjectives to indicate the opposite quality:
Un amigo leal – un amigo desleal
Estoy contento – Estoy descontento
Usually for this purpose we use the prefix i–:
real – irreal
legal – ilegal
Another prefix that is rapidly gaining popularity, anti-, means "against". Together with the commonly used faros antiniebla (fog headlights) and seguro antirrobos (security from robbery) in modern Spanish this prefix transforms nouns into adjectives with the meaning "to be ideologically against something". Morphologically these adjectives do not change neither by gender, nor by number.
We can hear more often
Soy antisistema
Soy antimóvil
Soy antihijos
and even Soy antiPutin. Typical of colloquial speech, these forms have not yet been established in the written language, so you can meet them in separate spelling, or with a hyphen.