Children must be innately guided during language acquisition. Rule acquistion can be watched when children generalize their performance to create a word form that they have never heard. Children command not just rules but memorized words. The course of langauge development in this area can be explained as an interaction between the innate organization of langauge and the child's experience with parental speech. There is lots of empirical evidence to support this hypothesis.
Analogy also plays a clear role in language.