all of them ;) but really depends on your weakpoints... that is more of a subjective request so hard to answer. If just starting out though I would say the grammar lessons are great bc it really helped me separate the different parts of the stimulus
I find a huge component to Main Conclusion questions is referential phrasing, which JY talks about in the early grammar portion of the curriculum. Many times you’ll have to synthesize a conclusion from parts of multiple sentences.