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Derivational morphology forms lexical items from existing bases, changing meaning and sometimes lexical category. It is part of linguistic morphology, alongside inflection, which supplies grammatical forms of a lexeme. The distribution of an affix matters as well as its visible spelling.^[ref]

For example, adding -ness to the adjective kind gives the noun kindness; adding un- to kind gives another adjective, unkind. Combining the operations yields unkindness. A category-sensitive analysis first forms unkind, then attaches -ness: this use of un- selects an adjective, whereas kindness is already a noun. The intermediate structure explains a restriction that merely listing the three morphemes misses. Derivational morphology therefore connects reusable word-building rules to both syntax and semantics.

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]

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.

A lemma is a conventional citation form used to represent a lexical entry or a set of related grammatical forms. It differs from a lexeme, the abstract vocabulary item being represented, and from an arbitrary truncation of a word's spelling. The convention is language-specific.^[ref]

In a simple English analysis, walks, walked and walking can be assigned the lemma walk. The Universal Dependencies guidelines also distinguish inflection from derivational morphology: organizations has citation form organization, not organize or organ.

Context can be needed before the citation form is chosen. The spelling saw can name a tool or occur as a past form of see; a process that uses spelling alone cannot settle every such ambiguity. Recording the part of speech, intended word sense and the original occurrence makes a text corpus analysis more reproducible than retaining only the normalized label.

A lexeme is an abstract vocabulary item whose different grammatical forms are treated as instances of the same item. It separates lexical identity from the particular form occurring in an utterance. An inflectional description can therefore connect several forms without treating each as an unrelated entry.^[ref]

For example, walk, walks, walked and walking can belong to the lexeme conventionally represented as WALK. Their differences involve inflection; forming a new item through derivational morphology is a different relation. A lemma is the conventional citation form used to label an entry, rather than the entire abstraction being represented.

This distinction matters when counting a text corpus. Counts of the exact form walked answer a different question from counts pooling all the specified forms of WALK. The grouping procedure must also handle ambiguous forms: spelling alone need not determine which lexeme a use instantiates.

Linguistic morphology studies how words are structured from morphemes and how their forms relate to grammatical functions and meaning. It distinguishes reusable patterns from merely splitting a spelling into arbitrary pieces. Spoken and signed languages can both have morphological structure.^[ref]

For example, replayed can be analyzed as re- plus play plus -ed: repetition and past marking combine with a lexical base. The segmentation predicts relationships with play, replay and played, rather than only describing one isolated word. Inflection marks grammatical alternatives, while derivational morphology can form new lexical items. A computational analyzer can represent permitted combinations with a finite-state machine; an analysis is useful when it generalizes to new forms while rejecting combinations outside the specified grammar.

A noun is a part of speech whose members commonly name entities or concepts and head noun phrases. Meanings alone do not supply a complete classification: a noun can name an event or property as readily as a physical object. Distribution and inflection provide additional grammatical evidence.^[ref]

For example, explosion names an event, but behaves as a noun in the explosion and two explosions. Its connection with an event does not make it a verb; compare The shell exploded. Likewise, redness is a noun even though it names a property associated with the adjective red.

In Universal Dependencies, the tag NOUN is reserved for common nouns, with PROPN used for proper nouns. That distinction matters when reproducing an analysis: a count of every traditionally named noun is not necessarily a count of only tokens tagged NOUN.

A part of speech is a grammatical word category identified by patterns of distribution, combination and, where relevant, inflection. Categories such as noun, verb and adjective help describe what an expression can do in a construction. Meaning is evidence, but a rule equating a category with one broad kind of meaning is insufficient.

For example, record is a noun in the record and a verb in record the result. The same spelling can therefore require different labels when its context changes. A computational annotation scheme must state whether it assigns categories to lexical entries, individual occurrences or both.^[ref]

Universal Dependencies separates a broad part of speech label from additional grammatical features. Two forms can share the label NOUN while differing in grammatical number. Keeping category and features distinct makes an analysis of a text corpus reproducible without multiplying category labels for every possible form.

Verb regularization is a form of language change in which an exceptional verb form is replaced by one following a productive grammatical pattern. For English past forms, this can mean adopting the -ed pattern rather than retaining an inherited alternative. Historical change in a speech community is distinct from a child's occasional application of a pattern to an irregular verb.

A quantitative study by Erez Lieberman and collaborators tracked irregular verbs across historical stages and identified help among verbs that became regular.^[ref] A hypothetical series with regular forms in 12 of 40 past-tense uses at one stage and 32 of 40 at another would show the regular variant's proportion rising from $0.3$ to $0.8$. Those counts alone would not establish the cause: recording practices, speaker composition and competing variants must also be checked. Regularization describes a pattern of inflection, not a judgment that the newer form is intrinsically better.