By the time children start school, they have acquired the essential grammar of English. Their vocabulary will grow enormously after they start school but they have little grammar to learn. Such things such as the second and third forms of irregular verbs can, at this stage, be considered a matter of lexis. By the time they start school most children have acquired the knowledge of these forms of the common verbs they have already met, and they have enough 'grammar' wired into them to construct in a regular way the second and third forms of new verbs they meet. Some of them will learn, in time, that if they want to refer to 'smiting' someone in the past they will have to use the word 'smote', not 'smited'; some will never learn that, just as some will never know what 'defecation' means.
The 'grammar' that we attempt to teach children from the age of five on is the knowledge of how to speak one form of the language that is generally accepted as being appropriate for use in moderately civil surroundings in the part of the country in which they live, and how to put their thoughts into a form of writing that will be considered 'literate/educated' enough for them to obtain a job in which the ability to write in that way is considered important.
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