Help the detective complete her final report by choosing the grammatically correct word.
The suspect couldn't have stolen the diamond ___ he was locked in the police station at the time.
The correct answer is because.
We use the subordinating conjunction because to introduce an adverb clause of reason, which must contain a subject and a verb ("he was locked"). "Because of" and "due to" are prepositions that must be followed by a noun phrase, not a full clause. "Therefore" is an adverb that would require different punctuation (like a semicolon) to connect two independent clauses.
Conjunction
A conjunction is a word that connects other words, phrases, or clauses. English has two main types: coordinating conjunctions join units of equal weight (and, but, or, so, yet, for, nor — the FANBOYS), while subordinating conjunctions introduce dependent clauses (because, although, if, when, while, since, unless).
Conjunctions are how you build compound and complex sentences instead of stacking short ones. The choice of conjunction signals the relationship between the ideas — addition, contrast, cause, condition, time — so picking the right one shapes the whole meaning.
Dependent clause
A dependent clause (also called a subordinate clause) is a clause that has a subject and a verb but can't stand alone as a complete sentence. It needs an independent clause to attach to. Because I overslept is a dependent clause; I missed the bus because I overslept is a complete sentence.
Dependent clauses come in three main types: noun clauses (acting as a noun: I know that he likes me), adjective/relative clauses (modifying a noun: the dwarf who has no beard), and adverbial clauses (modifying a verb or whole clause: Wherever she goes). They're how English packs more information into a single sentence.
Clause
A clause is a grammatical unit built around a verb — typically a subject plus a predicate (She laughed; The manager approved the budget). Clauses come in two types: independent clauses stand alone as complete sentences; dependent clauses need an independent clause to make sense (Because I overslept — incomplete on its own).
Spotting clause boundaries is the foundation of correct punctuation. Once you can see where one clause ends and another begins, comma rules, run-on sentences, and complex sentence structure stop being mysteries.
Preposition
A preposition is a small word that links a noun or noun phrase to other parts of the sentence — usually marking time, place, or relationship: in, on, at, to, from, with, over, under, between, during. The book on the table, We met at noon, She lives in Berlin.
Prepositions are deceptively small. Their meaning shifts dramatically by collocation (depend on, good at, afraid of), and their choice rarely translates directly between languages. Picking the right preposition is one of the trickiest, most idiomatic-sounding parts of English.
Adverb
An adverb modifies a verb, an adjective, or another adverb — adding information about how, when, where, how often, or to what degree something happens: she sings beautifully, unbelievably fast, we go there often. Many adverbs end in -ly, but plenty don't (well, fast, hard, almost).
Adverbs matter because they're how you add nuance without piling on extra clauses. Used well, a single adverb can sharpen a vague sentence (she answered → she answered honestly), but misplace one and the meaning drifts in a way native speakers immediately notice.
B1 | Intermediate
B1 is the intermediate level in the CEFR framework — the point where you stop relying on memorised phrases and start handling everyday English independently. At B1 you can describe experiences, explain opinions, and follow most clear standard speech on familiar topics like work, travel, and hobbies.
Grammatically, B1 means combining tenses with precision, building complex sentences, and starting to use passive voice, modal verbs for necessity and possibility, and verb patterns (gerund vs. infinitive). Knowing your level shapes what you study next: pushing too far ahead frustrates you; staying below your level wastes time.
Difficulty: Medium
The Medium difficulty tag marks questions and challenges in the middle of the difficulty range — typically suitable for A2 to B1 learners. Expect a single rule with realistic distractors, longer sentences, and contexts where you have to think before answering rather than reading off the obvious choice.
Filter by Medium when you're past the absolute basics and ready to consolidate. It's the level where most lasting progress happens — easy enough that you can finish without exhausting concentration, hard enough that getting it right means you've actually understood.