The rather neat chart presented here conceals a number of outstanding problems in the analysis of English morphology. So far, we have only considered examples of English words in which the different morphemes are easily identifiable as separate elements. The inflectional morpheme -s is added to cat and we get the plural cats. What is the inflectional morpheme that makes sheep the plural of sheep, or men the plural of man? And if -al is the derivational suffix added to the stem institution to give us institutional, then can we take -al off the word legal to get the stem leg? Unfortunately, the answer is “No.”

There are other problematic cases, especially in the analysis of different languages, but the solutions to some of these problems are clearer in some instances than in others. For example, the relationship between law and legal is a reflection of the historical influence of different languages on English word forms. The modern form law is a result of a borrowing into Old English (lagu) from a Scandinavian source over 1,000 years ago. The modern word legal was borrowed about 500 years later from the Latin form legalis (“of the law”). Consequently, there is no derivational relationship between the noun law and the adjective legal in English, nor between the noun mouth (from Old English) and the adjective oral (a Latin borrowing). An extremely large number of English words owe their morphological patterning to languages like Latin and Greek. Consequently, a full description of English morphology will have to take account of both historical influences and the effect of borrowed elements.