The human brain/mind has evolved language as one of its skills.

The mind/brain has evolved to organize experience into particular patterns [schemas].

Examples:  objects and actions;
                       single, plural;
                            past, present, future;
                                 before, after;
                                      actual, potential;
                  cause, effect;
                       inside, outside
                            negation;
                                 smaller, same, bigger

All languages reflect those universal patterns.

Children’s brains are designed to construct language from their experiences.

Grammatical rules are the directions for changing utterances to reflect changes in meaning.

Children construct and apply the rules before they learn the exceptions to the rules of their specific language.

Children understand language before they can utter it.

Adults provide the experience from which children abstract the specifics of the language
they hear as part of those experiences.