The correct answers are:
I missed the launch because my spacesuit was at the dry cleaners.
When the reason clause comes after the main clause, no comma is needed before "because."
Since my spacesuit was at the dry cleaners, I missed the launch.
When the adverb clause of reason comes before the main clause, a comma is required after it.
The incorrect options:
"Because my spacesuit was at the dry cleaners I missed the launch." is wrong because a fronted adverb clause must be followed by a comma.
"I missed the launch, since, my spacesuit was at the dry cleaners." is wrong because "since" should not be surrounded by commas — it is a conjunction introducing a clause, not a parenthetical.
Clause
Clause vs phrase: a clause has a subject + verb (she runs); a phrase does not (in the morning, running fast). This is the first distinction to make when analysing sentence structure.
A clause is a grammatical unit built around a verb: independent clauses make complete sentences; dependent clauses attach to them as modifiers or complements.
Diagnostic: find the verb. If there's a subject doing or being something → clause. If there's no subject-verb pair → phrase.
Conjunction
Coordinating vs subordinating conjunction: coordinating (and, but, or) joins two elements of equal rank — clause + clause, noun + noun. Subordinating (because, although, if) makes one clause depend on the other. The test: remove the conjunction. If both halves still feel complete → coordinating. If one half collapses → subordinating.
Conjunctions are connecting words for clauses, phrases, and words. The choice between coordinating and subordinating determines whether you're building a compound or complex sentence.
Diagnostic: does the conjunction create a dependent clause? Yes → subordinating. Does it link equals? → coordinating.
Punctuation
Punctuation vs grammar: grammar governs word forms and order. Punctuation governs how you mark the structure on paper. You can have perfect grammar with wrong punctuation (comma splices in otherwise correct sentences), and you can have correct punctuation with broken grammar. They're parallel systems.
Punctuation = the system of marks that make written sentence structure visible: periods, commas, semicolons, colons, apostrophes, dashes, and quotation marks.
Diagnostic: if your grammar is correct but readers misparse your sentences → punctuation problem. If punctuation is fine but word forms/order are wrong → grammar problem.
Complex sentence
Complex vs compound sentence: a compound sentence links two equal independent clauses with and/but/or. A complex sentence links an independent clause with a subordinate (dependent) clause — one idea is the main point, the other is background.
A complex sentence = independent clause + dependent clause. The dependent clause adds time (when), reason (because), condition (if), or detail (who/which).
Diagnostic: are both halves able to stand alone? Yes → compound. Can only one stand alone? → complex.
Comma
Comma vs semicolon vs period: all three can appear between two complete thoughts. Comma + conjunction (I left, and she stayed). Semicolon alone (I left; she stayed). Period = full stop (I left. She stayed.). Using just a comma between two independent clauses without a conjunction is a comma splice — the most common comma error.
The comma ( , ) separates sentence parts: lists, non-essential info, introductory phrases, and clauses before coordinating conjunctions.
Diagnostic: are both sides complete sentences with no conjunction between them? Don't use a comma alone — upgrade to a semicolon or add a conjunction.
B2 | Upper Intermediate
B2 vs C1: B2 means effective communication on complex topics with some effort. C1 means effortless fluency with precise register control. If you can argue a point but still reach for words and make structural slips under pressure, you're B2.
B2 is the upper-intermediate CEFR level: mixed conditionals, complex passives, reported speech with backshift, participle clauses, and sustained written argument.
Diagnostic: does your writing read as "competent non-native" or "could be native"? The former → B2. The latter → C1.
Medium
Medium vs Easy: Easy has one obviously correct answer and clearly wrong distractors. Medium has one correct answer but plausible distractors — you need to actually know the rule, not just guess from sound.
The Medium tag filters for A2–B1 challenges with realistic difficulty: one rule per question, plausible alternatives, everyday contexts.
Diagnostic: if you're scoring 90%+ on Easy, move here. If you're below 60% on Medium, go back to Easy for that topic. Target 70–80% accuracy for maximum learning.