Inflection expresses grammatical distinctions through the forms of a lexeme, without normally creating a new lexical item. Grammatical number, grammatical tense and grammatical agreement can be expressed inflectionally, depending on the language. The distinctions and their overt expression must be kept separate: a grammatical meaning can instead be expressed by a separate word.[ref]
Essentials of Linguistics (2nd edition) by Catherine Anderson and others. §5.7, introduction, Number, Agreement, Tense and Aspect
In
one boat and two boats, the ending marks a number contrast while both forms belong to the same lexeme. The pair walk and walked similarly contrasts grammatical forms of a verb. By comparison, deriving kindness from kind creates a new lexical item through derivational morphology. A computational description can store the base together with grammatical features, then predict its surface form. Irregular forms show why that prediction cannot always be reduced to concatenating one universal ending.