As is often the case with derivational morphology, many English prefixes can only be added to bases of particular lexical categories (or "parts of speech").
For example, the prefix re- meaning "again, back" is only added to verb bases as in rebuild, reclaim, reuse, resell, re-evaluate, resettle.
In particular, using verbs describing an irreversible action produces words often considered nonsense, e.g. unkill, unspend, unlose, unring.
a- typically creates adjectives from noun and verb bases: blaze (noun/verb) > ablaze (adj).
[2] In some analytic frameworks, such neo-classical prefixes are excluded from analyses of English derivation on the grounds that they are not analyzable according to a mostly synchronic (that is, relatively productive or easily recognizable and relating to present-day idioms) English (that is, "native") basis.
[4] Similarly, pairs such as defend/defense and double (or duple)/duplicity are not considered morphologically related in Marchand's treatment of English word formation and are thus excluded too, though they are regarded as derivatives of the shared roots in Jespersen's and Koizul's, while in others, they may be seen as allomorphs or variants (like deep/depth, a pair formed of Germanic components).
An example of such a now native English prefix is co- as in co-worker, which is ultimately derived from the Latin prefix com- (with its allomorphs co-, col-, con-, and cor-); and ex- as in ex-soldier, which derives from the Latin ex-.