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
- ✅ I was tired, but I stayed. — coordinating (links two equal clauses)
- ✅ I stayed because I was needed. — subordinating (introduces dependent clause)
- ✅ Although it rained, we went out. — subordinating (front position)
- ❌ I was tired, because. — incomplete (subordinating conjunction needs a clause after it)
Conjunctions connect words, phrases, or clauses. Coordinating (and, but, or, so, yet, for, nor) join equals; subordinating (because, although, if, when, while) introduce dependent clauses.
Pattern: coordinating = equal partners, same grammatical weight. Subordinating = one clause depends on the other for its meaning.
Dependent clause
- ❌ Because I was tired. — fragment (dependent clause standing alone)
- ✅ I left early because I was tired. — attached to independent clause
- ✅ The man who called is my uncle. — relative clause (modifies man)
- ✅ If you're ready, let's go. — conditional dependent clause
A dependent clause has a subject + verb but cannot be a complete sentence. It starts with a subordinating word (because, if, when, although, who, which) and must attach to an independent clause.
Test: does the clause start with a subordinator and feel incomplete on its own? → dependent clause. On its own, it's a fragment — attach it to a main clause.
Clause
- I missed the bus. — ✅ independent clause (stands alone)
- Because I overslept. — ❌ fragment (dependent clause, can't stand alone)
- Because I overslept, I missed the bus. — ✅ dependent + independent = complete sentence
- I missed the bus, and I was late. — ✅ two independent clauses joined by and
A clause is a unit built around a verb with a subject. Independent = can stand alone. Dependent = needs an independent clause to complete it.
Test: does the group of words have a subject + verb AND can it be a sentence on its own? Yes → independent clause. Has a subject + verb but feels incomplete → dependent clause.
Preposition
- ✅ interested in — ❌ interested on
- ✅ good at football — ❌ good in football
- ✅ depend on — ❌ depend of
- ✅ arrive at the station — ❌ arrive to the station
Prepositions link nouns to the rest of the sentence: time (at 5pm), place (in London), manner (with care), abstract (afraid of). Most are idiomatic — the "correct" preposition must be memorised with each verb/adjective combination.
Rule: there is no universal rule. English prepositions are learned by combination: interested IN, good AT, depend ON, afraid OF. Your native language's equivalent will often mislead.
Adverb
- ✅ She sings beautifully — ❌ She sings beautiful
- ✅ He drives carefully — ❌ He drives careful
- ✅ They arrived late — ✅ a late train (same form, both roles)
- ✅ She works hard — ❌ She works hardly (different meaning!)
The -ly words are adverbs — they modify verbs, adjectives, or other adverbs, telling you how, when, where, or to what degree.
Pattern: most adjectives become adverbs by adding -ly, but watch the exceptions — fast, hard, late, well — that keep the same shape or change meaning entirely.
B1 | Intermediate
- ✅ If I had more time, I would travel more. — second conditional
- ✅ The bridge was built in 1920. — passive voice
- ✅ She said she was tired. — reported speech with backshift
- ✅ Although it rained, we enjoyed the trip. — complex sentence with concession
These are B1 patterns — the CEFR intermediate level. At B1 you link ideas, use passive voice, handle reported speech, and manage second conditional — enough for travel, work basics, and everyday independence.
Marker: if you can explain why something happened and follow a news story, you're B1.
Medium
- If I were you, I would apologise. — one rule (second conditional), but distractors like was tempt you
- Answers require active thought, not instant pattern recognition
- Vocabulary and context are realistic, not artificially simplified
- Usually tests one rule, but the wrong answers are plausible
Medium marks middle-difficulty challenges: A2–B1, one rule tested, but with realistic distractors that require genuine understanding.
Use "Medium" when Easy feels too obvious but Hard feels overwhelming. This is where most productive learning happens — the sweet spot of difficulty.