We also have to remember that grammar is re-invented every generation. All native speakers of any language learn it, not by being given a set of grammar rules to memorize and apply, but by listening to and imitating native speakers. Rules are formed by hearing the language being used. For example, children learning to speak English will hear words such as children and imitate them. At some time into the learning process a child may come up with a 'new' plural form for child such as chiles or even chillers. Parents will sometimes take this as a step backwards but, in reality it is a step forward since a rule has been created afresh in the child's mind. Understanding of exceptions comes later.
The rules of grammar created by the individual child are only later overlaid with learned or received rules from teachers. The extent to which these learned rules affect the language patterns of the individual learning them will vary from person to person and, possibly, according to how closely the rules that the child constructed, follow the rules that are laid down in the classroom.
A case in point would be the old prepostion at the end of a sentence rule. In many cases the word at the end of the sentence is not a preposition but a particle of a phrasal verb. For a long time, traditional grammars did not recognize the phrasal verb and even now it seems to be popular only in the teaching of English as a second language. This has resulted in an inability to even recognize the difference between the two structures. A child will learn the phrase, e.g., that is the old friend I ran into and apply that structure to the phrase that is the burning building I ran into. The former is a legitimate use of the phrasal verb, to run into meaning meet accidentally. The latter is an example of the preposition at the end of a sentence, but since traditional grammars did not recognize the phrasal verb for many years (I might even say centuries) the speaker is faced with the disconnect of seeing a part of his internalized grammar being declared wrong. Responses to this dilemma will be different for different individuals.
