The best evidence of allomorphs being morphologically conditioned is found in the variant of English language suffixes in plural which exist in the words "children" and "oxen" to cite two examples.
In these cases, the plural form of the word "child", for example changes the root altogether (this is when sounds are realized as allomorphs) by adding the letter "r" to it in order to convert it in plural to children. Therefore, no longer is "child" the root of the word, but "childr" is. This is the evidence of morphological conditioning: The root changes (becomes an allomorphic realization) as a result of the morphological addition of the suffix "-en".
People who struggle trying to understand allophorms would find them easier to understand if two granted paradigms are accepted as fact:
- First, this is not a common occurrence and typically the words, children, oxen and the archaic form of the word "brother" brethren are among the few recognized morphological conditionings that employ the -en allomorph. Some other morphological allomorphs are teeth, put, fish theses (Friedrich Alexander Universität.)
- Second, there are no linguistic patterns that repeat themselves in other words. The words in the five classes of morphological allomorphs are limited.
By definition, every time allomorphic realizatons change due to the addition of suffixes, the word is considered to be morphologically conditioned. Actor/actress; tiger/tigress are examples of allomorphs of words that are morphologically conditioned because the root orthography (spelling) changes to accommodate the suffix -ress; this creates an allomorphic (different) realization.