A reason clause states the cause behind an action or situation described in the main clause. English has several conjunctions that introduce a reason — because, since, as — each with slightly different shades of formality and emphasis, and a separate preposition, because of, that takes a noun rather than a clause. Reference grammars such as the Cambridge Grammar classify these among the clauses of cause and reason. For example, She stayed home because it was raining uses a full reason clause, while She stayed home because of the rain uses the preposition with a noun phrase instead. A common learner error is mixing the two, writing She stayed home because of it was raining, which is ungrammatical.
What Is a Reason Clause?
A reason clause is a subordinate clause introduced by a reason conjunction. It can come before or after the main clause, with a comma required only when the reason clause comes first.
- We stayed home because it was raining.
- Since it was raining, we stayed home.
- As she was tired, she went to bed early.
Formation Table
| Word | Type | Example | Notes |
|---|---|---|---|
| because | conjunction + clause | She left because she was ill. | Most direct; states the main reason. |
| since | conjunction + clause | Since you asked, I’ll explain. | Often introduces a reason already known to the listener; can also mean ‘from a point in time’. |
| as | conjunction + clause | As it was late, we went home. | Common at the start of a sentence; more formal/written; also means ‘while’ in other contexts. |
| because of | preposition + noun/-ing | The match was cancelled because of the rain. | Followed by a noun phrase, never by a full clause. |
| due to the fact that | formal conjunction phrase + clause | The flight was delayed due to the fact that of a technical fault. | Very formal; often replaceable by ‘because’ in less formal writing. |
Because vs. Because Of
The single most common error with reason clauses is confusing because (which needs a full clause with a subject and verb) with because of (which needs only a noun phrase).
- Because + clause: The game was postponed because the weather was terrible.
- Because of + noun: The game was postponed because of the terrible weather.
Common Mistakes
- Wrong: The trip was cancelled because of the flight was delayed. Correct: The trip was cancelled because the flight was delayed, or because of the flight delay. (‘because of’ cannot be followed by a clause with its own subject and verb)
- Wrong: Because it was raining. We stayed home. Correct: Because it was raining, we stayed home. (a reason clause alone, without a main clause, is a sentence fragment)
- Wrong: As I was walking to school when I saw the accident. Correct: As I was walking to school, I saw the accident. (here ‘as’ means ‘while’, describing simultaneous time, not reason — do not confuse the two uses of ‘as’)
- Wrong: I like her since she is kind. Correct: I like her because she is kind. (in this everyday, neutral context ‘because’ is more natural than ‘since’, which often implies the reason is already obvious or shared)
Practice Exercises
Grammar Quiz
Choose the correct reason word for each context sentence.
Cloze Dropdown
Select because, since, as, or because of to complete each gap.
Complete the Sentence
Add a reason clause that logically explains each sentence stem.
Match-Up
Match results to the reasons that caused them.
Flash Cards
Practise forming reason clauses with instant feedback.
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Frequently Asked Questions
What is a reason clause?
A reason clause is a subordinate clause that explains the cause of the situation described in the main clause. It is introduced by conjunctions such as because, since, and as: We stayed home because it was raining.
What is the difference between ‘because’ and ‘because of’?
‘Because’ is a conjunction and must be followed by a full clause with its own subject and verb: because the flight was delayed. ‘Because of’ is a preposition and must be followed by a noun phrase, not a clause: because of the flight delay.
Can I start a sentence with ‘because’?
Yes, as long as the reason clause is followed by a main clause: Because it was raining, we stayed home. A reason clause alone, without a main clause attached, is a sentence fragment and is considered an error in formal writing.
What is the difference between ‘because’ and ‘since’ as reason words?
Both introduce a reason, but ‘since’ often signals that the reason is already known, obvious, or shared between speaker and listener: Since you already know the rules, I won’t repeat them. ‘Because’ is more neutral and states the reason directly without this assumption.
Does ‘as’ always mean ‘because’?
No. ‘As’ can mean ‘because’ (As it was late, we went home) or ‘while/during the same time as’ (As I was walking to school, I saw an accident). Context determines the meaning, which is why ‘as’ for reason is more common in writing than in speech, where ambiguity is avoided by using ‘because’ or ‘since’.
Can ‘because of’ be followed by a gerund?
Yes. ‘Because of’ can be followed by a noun (because of the rain) or a gerund phrase (because of him arriving late). It cannot be followed by a finite clause with a conjugated verb and separate subject in standard usage — that requires ‘because’ instead.
Is ‘due to the fact that’ the same as ‘because’?
They mean the same thing, but ‘due to the fact that’ is considerably more formal and wordier. In most writing contexts, replacing it with the simpler ‘because’ produces clearer, more concise prose without any loss of meaning.
Do I need a comma before ‘because’?
Generally no, when the reason clause follows the main clause: We stayed home because it was raining (no comma). A comma is used when the reason clause comes first: Because it was raining, we stayed home. An exception is when omitting the comma could cause ambiguity about what is being negated.
What is the difference between a reason clause and a purpose clause?
A reason clause explains an existing cause (I studied hard because the exam was difficult). A purpose clause states an intended future goal (I studied hard to pass the exam). Reason looks backward to a cause; purpose looks forward to a hoped-for result.
Why are reason clauses important for exam writing?
Clearly linking causes to effects with a variety of reason connectors (because, since, as, due to, owing to) demonstrates cohesion and grammatical range, both assessed directly in IELTS and Cambridge writing tasks. Repeating ‘because’ throughout an essay is penalised for lack of lexical variety.