What reason do we have for believing that children as young as age 2 or 3 years understand certain grammatical rules?

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Studies have shown that children as young as age 2 or 3 years sometimes overgeneralize grammatical rules, indicating that they have a basic understanding of those rules. They may use irregular verb forms in the same way they use regular verb forms, for example, by saying "goed" instead of "went." This demonstrates awareness of a rule for marking past tense, even if they haven't learned all the exceptions yet. This shows that they understand grammatical rules at a basic level. However, they are not usually able to state the rules, ask for corrections or correct others at this age.

What Reason Do We Have for Believing That Children as Young