Inflection
Inflection supplies grammatical forms of a lexeme without normally making a new dictionary entry: for example a plural, a case form or a finite verb form. Its actual inventory is language-specific. A corpus token can be inflected even if its surface shape happens to match the citation form.
Derivation
Derivation creates a related lexical item, often with a new meaning or word class. The boundary is not always sharp. Ask whether the operation is productive, whether the meaning is predictable, whether it changes syntactic distribution, and whether speakers treat the result as a separate lexeme. Dictionary headwords and grammar labels are hypotheses about that organization.
Historical comparison
Do not merge forms from centuries apart into one synchronically valid paradigm without an argument. A later spelling may reflect sound change rather than a new suffix; a formulaic genre may preserve an older form. Record every token with date, dialect, witness and edition before filling a comparative table.
Modern-language examples on this page clarify a linguistic pattern. They do not establish unattested sounds or forms for an ancient language. Follow the cited sources below and keep editorial readings separate from the surviving trace.