Prepositions of time and place (in, on, at), preposition collocations that go with particular verbs and adjectives, telling a preposition from a conjunction or an adverb particle, and the fixed pairs the exams test again and again.
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.