The word rule is actually rather ambiguous, and can cause some confusion when used to describe language learning and use.
The video clip in Fig. 1.6 below explains the ambiguity.
Fig. 1.6 Rules of English
Rules viewed as regulations are external to those who abide by them; they mandate correct, authorised, acceptable behaviours. Rules viewed as regularities, on the other hand, capture actual patterns of behaviour, and can be inferred to operate in the personal histories of individuals, guiding what they do, but through custom rather than obligation.