To inflect a word is to change its form in some way, usually fairly small, to use it in a different grammatical context. A noun, for example, can be inflected to show that it is plural - regular nouns do this by adding '-s'. A verb may be inflected to show tense: the regular inflection for the past tense in English is the addition of '-ed'.
Some languages have many inflections: some have very few. English has perhaps fewer inflections than any other west European language. This is one reason why people sometimes say that "English has not got very much grammar". This is not the whole truth. English does not have many inflections ('accidence' in older terminology); but it does have much subtlety in the relationships which each word has with the others in the sentence ('syntax').
The study of the changes in the shapes of words for grammatical reasons is nowadays called inflectional morphology. Such particles as '-ed', for tense, '-s', for plural in the noun, and '-er', for comparison in the adjective and adverb, are known as inflectional morphemes.
- Some other changes in the forms of words are studied under derivational morphology.