Morphology is the study of word forms. A program called a morphological analyzer will convert words to root forms and affixes (prefixes and suffixes); the root forms can then be looked up in the lexicon.
For English, a fairly simple suffix-stripping algorithm plus a small list of irregular forms will suffice.[Winograd, T., in Understanding Natural Language, Academic Press, 1972, presents a simple algorithm for suffix stripping. A thorough treatment can be found in Slocum, J., ``An English Affix Analyzer with Intermediate Dictionary Lookup'', Technical Report LRC-81-01, Linguistics Research Center, University of Texas at Austin, 1981.]
running --> run + ing went --> go + ed
If the lexicon needed for an application is small, all word forms can be stored together with the root form and affixes. For larger lexicons, a morphological analyzer would be more efficient. In our discussions of syntax, we will assume that morphological analysis has already been done.