linguistics1-10  Exercise 1: Overgeneralisation
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"I goed to the shop and bringed two mouses"

  goed      go -> went is irregular. The child has applied the regular past
            rule (add -ed) to "go".
  bringed   bring -> brought is irregular. Same rule, applied too widely.
  mouses    mouse -> mice is irregular. The child has applied the regular
            plural rule (add -s).

Why this is evidence for rule-building:

  Adults do not say "goed", "bringed" or "mouses", so the child cannot have
  copied them. They must have worked out a general rule from the regular
  verbs and nouns and then used it for new cases. The errors disappear when
  the child learns the exceptions. Children also add the plural to nonsense
  words (the "wug" test, Chapter 3), which no one could have taught them.