When I was a missionary in Prague, I was over-whelmed with learning the fourth most difficult language (after Navaho, Apache, and Japanese). I devised a number of language lessons to combat this problem, which I later presented as PEBMSM, the phrase-exemplar based multi-sensory method.
I've given this presentation to several different ESL audiences. Although it utilizes the Czech example, the presentation argues that one learning strategy, music, can activate up to 11 more. Many can see in PEBMSM the application of a theoretical basis for building musical interventions in ESL.
One reason why grammar is so difficult is that we approach it through rules. However, the linguistic intelligence does not work according to rules, per se. In fact, it generates options through analogical comparison with other phrases. Grammar develops over time through input to the passive mind.