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
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.
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.
Punctuation
Punctuation is the set of visual marks — periods, commas, question marks, colons, semicolons, apostrophes, quotation marks, hyphens and dashes — that show readers where sentences begin and end, where pauses go, and how parts of a sentence relate.
Punctuation does two jobs: it follows the rhythm of speech (where you'd pause aloud) and it marks the structure of clauses. Mismatch the two and writing reads either as breathless or as choppy. Mastering the basics is a small investment with huge returns — clear punctuation makes prose look careful and considered.
Complex sentence
A complex sentence combines an independent clause with at least one dependent (subordinate) clause: I missed the bus because I overslept. The dependent clause adds extra information — usually about time, reason, condition, or which thing is meant — but can't stand alone. It's introduced by a subordinating conjunction (because, although, if, when, while) or a relative pronoun (who, which, that).
Mastering complex sentences is the move from simple, choppy writing to prose that links ideas. It's also where comma decisions get interesting — placement depends on which clause comes first.
Comma
The comma ( , ) is the most-used punctuation mark in English, separating parts of a sentence where the reader needs a small pause without a full stop. Its main jobs: separating items in a list (apples, pears, and figs), marking off non-essential information (My brother, who lives in Paris, called), and joining clauses with a coordinating conjunction (I went home, and she stayed).
Misuse of the comma — too many, too few, or in the wrong place — is the single most common punctuation issue in English writing. Get it under control and your sentences immediately read more cleanly.
B2 | Upper Intermediate
B2 is the upper-intermediate level in the CEFR framework, sitting between B1 and C1. At B2 you can read editorials, follow most TED talks without subtitles, and hold extended conversations on abstract topics — including topics outside your everyday life.
Grammatically, B2 means flexible control of mixed conditionals, passive voice across tenses, reported speech with proper backshifting, and participle clauses. B2 is the standard target for university entrance exams (IELTS 5.5–6.5, TOEFL 87–109) and most skilled-migration thresholds — knowing whether you're there shapes your study plan.
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.