In which stage do children typically begin to use uncontractible auxiliary verbs?

Children typically begin to use uncontractible auxiliary verbs during Brown's Stage 5. At this stage, which generally occurs between ages 3.5 and 4 years, children are expanding their grammatical skills and beginning to master more complex sentence structures. They develop a better understanding of verb forms and auxiliary verbs, including the use of uncontractible forms like "is" and "are" when they help form the tense in a sentence.

The use of uncontractible auxiliary verbs indicates a significant advancement in language development, illustrating that children are not only using verbs but also grasping how to apply various verb tenses and structures. Each stage in Brown's typology represents developmental milestones in morphological and syntactic usage, with Stage 5 showcasing the child’s growing ability to manipulate language more flexibly and accurately.