I am writing to ask for a three-day extension as my laptop spontaneously combusted last night.
"As" introduces the adverb clause of reason. You cannot use "because of" or "due to" here because the reason is a complete clause ("my laptop spontaneously combusted"), not a simple noun phrase.
Because I lost all my files in the small fire, I have to start the essay completely from scratch.
"Because" is the correct conjunction to introduce a full dependent clause.
I hope you can grant me this extra time, since this was a completely unforeseeable disaster!
"Since" is used here to introduce a reason that is already somewhat understood or logically follows from the previous context.
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.
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.
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.
Phrase
In grammar, a phrase is a group of words (sometimes a single word) that functions as a single unit in a sentence — but doesn't include a subject + verb pair the way a clause does. Common types: noun phrase (the old red car), verb phrase (has been running), prepositional phrase (on the table), adjective phrase (incredibly tired), adverb phrase (very quickly).
Phrases are the building blocks between individual words and full clauses. Recognising them helps you see how sentences hold together — and where you can break, expand, or rearrange them without losing meaning.
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.
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.