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Morpheme #14 of 14Brown's Stage V+30–50 months

Contractible Auxiliary

Contractible auxiliary 'be' is the fourteenth and last of Brown's 14 morphemes, mastered between 30 and 50 months and the last to reach criterion.

At a glance

Acquisition order
#14 of 14 (Brown 1973)
Symbol / form
's/'re (auxiliary)
Brown's stage
V+
Typical age of mastery
30–50 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

Contractible auxiliary "be" is the reducible form of auxiliary "be" used in present progressive and passive constructions where contraction is grammatically licensed ("he's running", "they're working", "she's being chased"). Brown placed it last in his 14 because its mastery comes after every other morpheme in his order and represents the closure of the present-tense "be" verb system. Once the child controls contractible auxiliary in connected speech, the entire Brown morpheme set has been acquired and the child has reached late Stage V+ in Brown's stage framework.

Acquisition trajectory

Children begin producing contractible auxiliary sporadically in late Stage IV and reach 90% mastery in obligatory contexts by approximately 50 months in typical development. The acquisition order — copula before auxiliary, uncontractible before contractible — produces the canonical four-way ordering: uncontractible copula → contractible copula → uncontractible auxiliary → contractible auxiliary. Brown observed this ordering in all three of his longitudinal subjects and it has been replicated in dozens of cross-sectional studies since. The first productive contracted auxiliary forms appear on high-frequency hosts paired with high-frequency present-progressive verbs ("he's running", "she's playing"). Children continue to produce the full form ("he is running") in parallel for many months without signalling a problem, and dialectal variation in casual speech permits both forms in the same conversation throughout life.

Examples in obligatory context

These are the kinds of child utterances a clinician would code as a correct production of 's/'re (auxiliary) when scoring a language sample.

  • “He's running.”
  • “She's playing outside.”
  • “They're coming home.”
  • “I'm reading a book.”
  • “It's raining hard.”
  • “We're going to the store.”

Common errors during the acquisition window

  • Omission of the auxiliary entirely: "he running" for "he's running"
  • Failure to inflect the auxiliary for person: "he are running" for "he's running"
  • Use of bare stem auxiliary: "he be running"
  • Inconsistent marking across present-progressive verbs 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

Contractible auxiliary is the canonical end-state marker for typical grammatical development. A child who has mastered contractible auxiliary in connected speech has by definition mastered the entire Brown morpheme set and is in late Stage V+. Score contractible auxiliary in a language sample by identifying every obligatory present-progressive context where the auxiliary surface would license contraction and coding each as correct (full or contracted), omitted, or substituted. Children with DLD show prolonged omission of contractible auxiliary into the school years, often well past the age at which they have mastered every other morpheme in the 14, and the persistent gap is one of the most reliable single markers of the optional-infinitive profile in the older school-age range. Pair the score with the other three "be" verb morphemes for a complete picture.

“When a school-age child finally closes the contractible auxiliary gap, you have crossed a real grammatical threshold. Document it in the IEP report as a milestone, not a footnote.”
The last gap to close

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.

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Brown'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.

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DSS 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.

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Related morphemes

#13 · Stage V+

Contractible Copula

Contractible copula 'be' is the thirteenth of Brown's 14 morphemes, mastered between 29 and 49 months in obligatory contractible contexts.

#12 · Stage V

Uncontractible Auxiliary

Uncontractible auxiliary 'be' is the twelfth of Brown's 14 morphemes, mastered between 29 and 48 months in obligatory non-contractible contexts.

#7 · Stage III

Uncontractible Copula

Uncontractible copula 'be' is the seventh of Brown's 14 morphemes, mastered between 27 and 39 months in obligatory non-contractible contexts.

References

  1. Brown, R. (1973). A First Language: The Early Stages. Harvard University Press, pp. 367–388.
  2. Cleave, P. L., & Rice, M. L. (1997). An examination of the morpheme BE in children with specific language impairment: The role of contractibility and grammatical form class. JSLHR, 40(3), 480–492.
  3. 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.