Subordinating conjunctions are words that join a subordinate clause to a main clause and show how the two ideas relate. They can express time, cause, condition, contrast, or reported content. In English grammar, they are studied alongside Conjunctions and Clauses, because their job is to build larger sentence patterns from smaller clause units.

A subordinating conjunction introduces a clause that cannot stand alone as a complete sentence. The clause it introduces depends on the main clause for full meaning. This dependency is what separates subordinating conjunctions from coordinating conjunctions and helps define clause structure in English.

Idea	Example
A subordinating conjunction attaches a dependent clause to a main clause.	Although it rained, we walked.
The dependent clause adds information but does not complete the sentence alone.	Because she was late, the meeting started without her.
The conjunction shows the relationship between the two clauses.	If you are ready, we can begin.

The most common subordinators include although, because, since, if, when, while, until, after, before, and unless. These words introduce clauses of time, cause, condition, and contrast, and their meaning becomes clear from the sentence context. Since can refer to time or cause, and informal speech may also use like as a conjunction, though that use is nonstandard.

Idea	Example
Although shows contrast.	Although it was cold, they went outside.
Because shows cause.	We stayed home because it was raining.
Since can show time or cause.	Since you arrived, things have been easier.
If shows condition.	If he calls, tell me.
When shows time.	When the bell rings, class begins.
Until shows a limit in time.	Wait until I finish.
Unless shows an exception.	You cannot enter unless you have a ticket.

Subordinating conjunctions introduce two broad clause types. Adverbial clauses modify the main clause by giving information about time, cause, condition, or contrast. Noun clauses function like nouns and often begin with that, whether, or if when they report thoughts, beliefs, or other content.

Idea	Example
Adverbial clauses can show time.	We left after the movie ended.
Adverbial clauses can show cause.	She smiled because she understood.
Noun clauses can act as objects.	I know that she left.
Noun clauses can follow verbs of thinking or reporting.	He asked whether the train was late.

When a subordinate clause comes first, English normally uses a comma before the main clause. When the subordinate clause follows the main clause, a comma is usually not needed. In careful writing, this punctuation helps readers see where the dependent clause ends and the main clause begins, although native speakers sometimes omit commas in informal writing.

Idea	Example
Use a comma after a fronted subordinate clause.	Because it rained, we stayed inside.
Do not usually use a comma when the subordinate clause follows.	We stayed inside because it rained.
The comma marks the boundary between clauses.	If you are tired, you should rest.

Subordinate clauses normally keep standard subject verb order. They do not use the inversion patterns that appear in questions. This means the clause begins with the subordinator, followed by the subject and then the verb, as in if he comes or when she arrives.

Idea	Example
Subordinate clauses keep normal subject verb order.	If he comes, we will start.
Subordinate clauses do not use question order.	When she arrives, we will leave.
The subordinator comes before the subject.	Although they were tired, they continued.

In many noun clauses, that can be omitted in informal speech and writing when the meaning remains clear. The clause still functions as a noun clause even without the word that. In more formal styles, writers often keep that to make the structure easier to read.

Idea	Example
That can often be omitted in informal noun clauses.	I think she is right.
Keeping that can sound more formal or clearer.	I think that she is right.
The meaning stays the same when the word is dropped.	She said he was busy.

Some adverbial clauses can be reduced to participle phrases when the subject is the same and the meaning is clear. This reduction often appears with time or simultaneous action. A full clause such as while she was walking can become a shorter form such as walking, and the reduced form still shows the adverbial relation.

Idea	Example
An adverbial clause can reduce to a participle phrase.	Walking down the street, I saw him.
Reduced forms often show simultaneous action.	While walking, she listened to music.
Reduction keeps the adverbial meaning.	After finishing dinner, they left.

The tense inside a subordinate clause often shows time relative to the main clause, which is part of sequence of tenses. A past form in the subordinate clause may refer to an earlier event, while a present or future meaning depends on the surrounding clause. This is especially important in reported content and other noun clauses, where the clause relationship affects interpretation.

Idea	Example
The subordinate clause can show earlier time.	I knew that she had left.
The clause tense is relative to the main clause.	When he arrived, we had already eaten.
Reported content often follows sequence of tenses.	She said that she was tired.

Subordinating conjunctions differ from coordinating conjunctions because they create dependence rather than linking equal units. They also differ from relative pronouns because relatives usually modify nouns, while subordinators introduce full dependent clauses with adverbial or noun clause roles. These differences are central in Coordinating Conjunctions and Correlative Conjunctions.

Idea	Example
Coordinators join equal elements.	I stayed, and she left.
Subordinators make one clause depend on another.	I stayed because she left.
Relative words modify a noun rather than join clauses as dependents.	The book that you chose is helpful.

Subordinating conjunctions introduce dependent clauses that add time, cause, condition, contrast, or reported content to a main clause. They commonly appear in adverbial clauses and noun clauses, they normally keep standard subject verb order, and they often require a comma when the subordinate clause comes first. Their forms and punctuation patterns make them essential for sentence structure, clause identification, and accurate written English.