The sentence is describing a past action that happened to a book, the correct answer is sat, as it is the past simple form of the verb "to sit" indicating that the book was in a particular position in the past. The relative clause "which sat on the top shelf" describes the book and gives more information about its location.
Clause
If you've ever been told your sentence is a "run-on" or that you've used a comma where there should be a semicolon — you've hit the limits of writing without seeing clauses. Get this concept solid and most punctuation problems quietly disappear: you can finally tell a complete thought from a fragment without guessing.
A clause is a grammatical unit built around a verb, usually with a subject and a predicate. Independent clauses stand alone (I missed the bus); dependent clauses can't (Because I overslept). Combine them and you build complex sentences.
Simple tense
If you're at A1/A2 and the array of English tenses feels overwhelming, here's the good news: most of what you need to say at the start fits in the simple forms. I work, I worked, I will work — three forms cover habits, completed past actions, and basic future. Master these first; the progressive and perfect come more easily once the simple is solid.
The simple aspect is the unmarked verb form — no progressive -ing, no have + past participle. I go, I went, I will go. Marks single completed actions, habits, or permanent states.
Relative clause
If you've ever paused over who vs whom vs that vs which — or wondered whether a comma belongs before who — you've hit the relative-clause puzzle. English makes meaning depend on whether the clause is essential information or just extra; one missing comma can flip the meaning of the whole sentence.
A relative clause is a dependent clause modifying a noun, introduced by a relative pronoun (who, whom, whose, which, that) or relative adverb (where, when, why). Restrictive relatives are essential and unmarked; non-restrictive are extra information and set off with commas.
Past tense
If you've ever told a story in English and felt the timeline get tangled — I came home, the dog ate, the cat slept — you've hit the limits of using simple past for everything. The past tense system has four forms specifically because real stories have layered timing: things that happened before other things, actions caught in progress, sequences of completed events.
The past tense has four English forms: simple past (I walked), past progressive (I was walking), past perfect (I had walked — earlier than another past event), past perfect progressive (I had been walking — ongoing up to a past point). Plus irregular verbs for the simple-past form.
B2 | Upper Intermediate
If a university admissions team or visa office has ever asked you for an English test score, B2 is probably the level they had in mind. It's the threshold where your English stops being a constraint and starts being a tool — and the line between B1 and B2 is often the line between "stuck in beginner classes" and "ready to study or work in English."
B2 is the upper-intermediate level in the CEFR framework, demanding flexible control of mixed conditionals, passive voice across tenses, reported speech with backshifting, and participle clauses.
Difficulty: Medium
If easy questions feel too obvious but hard questions leave you guessing, you're probably ready for Medium — the level where most real learning happens. It pushes just enough to expose the rules you don't quite have yet, without burying you in edge cases. This is where steady fluency is built, one well-aimed challenge at a time.
The Medium difficulty tag marks middle-range challenges — typically A2 to B1. One rule per question, realistic distractors, and contexts that require active thought rather than instant recognition.