Word formation: affixation

Infixes

There is question as to whether the limited usage of infixation in English actually a morphological process since the word being inserted is not itself an infix, as it is free-standing and not a bound morpheme.    Furthermore, there is no resulting derivation or inflection.

Only expletives are used as infixes and in only a limited  number of words.  For example, infixes are only permitted when the expletive is flanked by stress.  This means that only words with initial stress (trochees and not iambs) will be candidates for infixation.
(2) 
un-expletive-believable  but  *unbe-expletive-lievable

Clitics

Clitics are unstressed reduced units of meaning that attached to a limited number of host words.  They generally are not considered a type of affix since they do not meet specific minimal phonological requirements (which will not be discussed here).  Proclitics attach to the beginning of a root, e.g., ‘tis for ‘it is’, ‘dyou for ‘do you’. Enclitics are attached word finally, e.g., what’s for ‘what is’.