Regular Third-Person Singular -s
Regular third-person singular -s is the tenth of Brown's 14 morphemes, marking present-tense agreement and reaching mastery between 26 and 46 months.
At a glance
- Acquisition order
- #10 of 14 (Brown 1973)
- Symbol / form
- 3sg -s
- Brown's stage
- IV
- Typical age of mastery
- 26–46 months
Mastery is defined as correct use in 90% of obligatory contexts across three consecutive language samples (Brown 1973). Individual variation is wide — these ranges describe typically developing English-speaking children and should be cross-referenced against the primary literature before clinical use.
What this morpheme is
Regular third-person singular -s is the bound suffix that marks present-tense agreement with a third-person singular subject ("she runs", "he eats", "the dog barks"). It is the only inflectional agreement marker remaining in modern English present-tense paradigms — a relic of a much richer Old English system — and as a result it carries a heavy diagnostic load in the developmental literature. Brown placed it tenth in his order, in late Stage IV, because the morpheme requires the child to track both the tense and the person/number features of the subject simultaneously.
Acquisition trajectory
Children begin producing third-person singular -s sporadically in late Stage III and reach 90% mastery in obligatory contexts by approximately 46 months in typical development. The acquisition is slower and more uneven than the plural -s suffix despite the two morphemes sharing identical phonological form. Brown noted that children frequently produce plural -s correctly months before they apply third-person -s in present-tense contexts, which provides strong evidence that the two suffixes are stored as different grammatical entries even though they sound identical. The error pattern is overwhelmingly bare-stem omission ("she run", "he eat") rather than substitution; overgeneralisation to other person/number contexts ("I runs", "they eats") is rare in typical development and worth flagging when it appears. Children with DLD show prolonged omission well into the school years, often with no overgeneralisation phase at all.
Examples in obligatory context
These are the kinds of child utterances a clinician would code as a correct production of 3sg -s when scoring a language sample.
- “She runs fast.”
- “He eats cookies.”
- “The dog barks at me.”
- “It goes here.”
- “Mommy works at the store.”
- “My sister plays soccer.”
Common errors during the acquisition window
- Bare stem in obligatory context: "she run" for "she runs"
- Substitution to other person/number: "I runs" or "they runs"
- Failure to control the syllabic /əz/ allomorph: "she catch" for "she catches"
- Inconsistent marking across the same verb in the same sample
Many of these errors are developmentally normal during the acquisition window. The clinical signal is persistence past the typical age of mastery, not the presence of any single error in early production.
Clinical relevance and scoring
Regular third-person singular -s is, alongside regular past -ed, the most diagnostically informative morpheme in the 14 for childhood DLD. The Rice-Wexler tense-marking account predicts prolonged bare-stem use in obligatory third-person contexts, and large studies have replicated the finding consistently. A school-age child who produces "she run" and "he eat" in connected speech is showing the canonical clinical pattern. Score third-person singular -s in a language sample by identifying every obligatory present-tense context with a third-person singular subject and coding for correct production, bare stem, or substitution. Pair the score with the regular past -ed score to compute a composite tense-marking percentage that is more diagnostically robust than either morpheme alone.
“Third-person -s and regular past -ed move together. Score them as a pair in your composite tense-marking percentage and the clinical signal jumps a whole standard deviation.”
Get the full analysis
Score 3sg -s automatically with ConductSpeech
Upload the audio. ConductSpeech transcribes the language sample, identifies every obligatory context for 3sg -s, computes the percentage correct, and writes a parent- and team-ready summary in minutes.
Free tools for scoring this morpheme
MLU Calculator
Paste a language sample and get Mean Length of Utterance in morphemes and words, total utterances, total morphemes, and the matching Brown's stage. Implements Brown (1973) morpheme counting rules and runs entirely in your browser.
Open toolBrown's Stages Lookup
Interactive reference for Brown's (1973) five stages of grammatical development. Look up a stage by child age or MLU, see the MLU range, acquired morphemes, example utterances, and clinical milestones for each stage.
Open toolDSS Calculator
Free Developmental Sentence Scoring (DSS) calculator for speech-language pathologists. Paste 50 sentences, tap weighted points across the eight Lee (1974) grammatical categories — Indefinite Pronouns, Personal Pronouns, Main Verbs, Secondary Verbs, Negatives, Conjunctions, Interrogative Reversals, Wh-Questions — plus the all-correct sentence point, and the calculator returns the live DSS with a per-category breakdown. Mobile-friendly tap-through grid, client-side, no sign-up.
Open toolRelated morphemes
Regular Past -ed
Regular past tense -ed is the ninth of Brown's 14 morphemes, marking past on regular verbs and reaching mastery between 26 and 48 months.
#11 · Stage VIrregular Third-Person Singular
Irregular third-person singular forms (does, has) are the eleventh of Brown's 14 morphemes, mastered between 28 and 50 months.
#5 · Stage IIIrregular Past Tense
Irregular past-tense verbs ("went", "ate", "broke") are the fifth of Brown's 14 morphemes, mastered between 25 and 46 months in typical development.
References
- Brown, R. (1973). A First Language: The Early Stages. Harvard University Press, pp. 254–271.
- Rice, M. L., Wexler, K., & Hershberger, S. (1998). Tense over time: The longitudinal course of tense acquisition in children with specific language impairment. JSLHR, 41(6), 1412–1431.
- Bedore, L. M., & Leonard, L. B. (1998). Specific language impairment and grammatical morphology: A discriminant function analysis. JSLHR, 41(5), 1185–1192.