Putting items that play the same role into the same grammatical form — in a list, after a correlative pair, in a comparison, and after a single governing word — and spotting the faulty parallelism that error-detection and sentence-improvement questions rely on.
Reading the 9 sections above, the worked examples and the practice set is free — the whole knowledge base is on the free plan, along with flashcards, private notes and Ask Onely.