Commas

You already know how to use commas in lists, after introductory words, and with direct address. Now you will learn how to use commas in compound sentences and complex sentences to make your writing clearer and more interesting.

What You'll Learn

How to use commas in compound sentences (two complete ideas joined by a conjunction)
How to use commas in complex sentences (a main clause and a subordinate clause)
How to decide whether a comma is needed based on the order of clauses
How to avoid common comma errors in longer sentences

When to Use

In compound sentences: "The sky turned dark**,** and we hurried home."
In complex sentences with the subordinate clause first: "Although it was a holiday**,** Father still went to work."
After a long introductory phrase: "During our trip to the Singapore Zoo, we saw many animals."
Before a conjunction that joins two complete sentences: "She wanted to buy the book**,** but she did not have enough money."
When a subordinate clause interrupts the main clause: "The boy**,** who was standing near the door**,** waved at us."

How to Form

Commas in Compound Sentences

A compound sentence joins two complete sentences (main clauses) with a coordinating conjunction. Place a comma before the conjunction.

Conjunction	What it does	Example
and	adds an idea	I packed my lunch, and my sister packed hers.
but	shows a difference	The food was delicious, but it was too spicy for me.
or	gives a choice	We can take the MRT, or we can walk to the hawker centre.
so	shows a result	It started to rain, so the pupils stayed in the classroom.

Pattern: Main clause , conjunction main clause.

Commas in Complex Sentences

A complex sentence has a main clause and a subordinate clause joined by a subordinating conjunction. The comma rule depends on which clause comes first.

Order	Comma?	Example
Main clause first	Usually no comma	She smiled because she received a gold star.
Subordinate clause first	Comma after subordinate clause	Because she received a gold star, she smiled.

Common Subordinating Conjunctions

Type	Conjunctions
Time	when, while, before, after, until, as soon as
Reason	because, since, as
Condition	if, unless
Contrast	although, even though, though
Purpose	so that, in order that

Key Rules

Comma before a coordinating conjunction joining two complete sentences: When and, but, or, or so joins two sentences that can each stand alone, place a comma before the conjunction. "The teacher smiled**,** and the class cheered."
No comma when a conjunction joins only words or phrases: If only words, phrases, or verbs are joined, do not add a comma. "She likes reading and drawing." (no comma -- only two activities, not two complete sentences)
Comma when a subordinate clause comes first: When a sentence begins with a subordinating conjunction, put a comma after the subordinate clause. "When the bell rang**,** the children rushed out."
Usually no comma when the main clause comes first: When the main clause comes before the subordinate clause, you usually do not need a comma. "The children rushed out when the bell rang."
Use two commas for an interrupting clause: When extra information appears in the middle of a sentence, put a comma before and after it. "My neighbour**,** who moved here last year**,** is very friendly."
Do not mix up compound and complex sentence comma rules: In a compound sentence, the comma goes before the conjunction. In a complex sentence with the subordinate clause first, the comma goes after the subordinate clause.

Common Mistakes

Wrong	Right	Why
I wanted to go out but it was raining.	I wanted to go out, but it was raining.	Two complete sentences joined by "but" need a comma before the conjunction
She studied hard, and passed the test.	She studied hard and passed the test.	"Passed the test" is not a complete sentence on its own, so no comma is needed
Although it was cold we went swimming.	Although it was cold, we went swimming.	Subordinate clause comes first, so a comma is needed after it
We went swimming, because it was hot.	We went swimming because it was hot.	Main clause comes first, so no comma is needed before "because"
The girl who won the race, is my classmate.	The girl who won the race is my classmate.	"Who won the race" identifies which girl -- it is essential information, so no commas
My uncle, took us to East Coast Park, and we flew kites.	My uncle took us to East Coast Park, and we flew kites.	Do not put a comma after the subject; the comma goes before "and" because it joins two complete sentences

Clue Words

Coordinating conjunctions (comma before these when joining two complete sentences)

and, but, or, so

Subordinating conjunctions (comma after the clause if it comes first)

when, while, before, after, until, because, since, if, unless, although, even though, so that

Interrupting clause signals (two commas around the extra information)

who, which, where

Tip: Use the "split test" -- cover the conjunction and read each side separately. If both sides are complete sentences, you need a comma before the conjunction. If only one side is complete, you probably do not need a comma.

Practice Tips

The split test for compound sentences: Cover the conjunction (and, but, or, so) and read each side on its own. If both sides are complete sentences, place a comma before the conjunction. If one side cannot stand alone, no comma is needed.
The flip test for complex sentences: Move the subordinate clause to the front of the sentence. If it sounds right with a pause after it, add a comma. Then move it to the back -- you can usually remove the comma.
Read aloud: Read your sentence out loud slowly. If you hear a natural pause between two complete ideas or after an opening clause, that is where the comma belongs.
Check for false commas: After writing, look for commas that separate a subject from its verb (wrong) or commas before "because" when the main clause comes first (usually wrong). Remove them.

Quick Reference

Sentence Type	Comma Rule	Example
Compound (two main clauses)	Comma before the conjunction	I finished my work, and I went out to play.
Compound (joining words only)	No comma	She likes drawing and painting.
Complex (subordinate clause first)	Comma after the subordinate clause	Before the match started, the coach gave us a pep talk.
Complex (main clause first)	Usually no comma	The coach gave us a pep talk before the match started.
Interrupting clause	Two commas around the extra information	The library, which is near our school, has many books.

What to Check	Ask Yourself
Compound sentence comma	Can both sides stand alone as complete sentences? If yes, add a comma before the conjunction.
Complex sentence comma	Does the sentence start with a subordinating conjunction? If yes, add a comma after the subordinate clause.
No comma needed	Am I only joining two words, phrases, or verbs? If yes, skip the comma.
Interrupting clause	Is there extra (non-essential) information in the middle? If yes, put commas before and after it.

Commas (P4) (Primary 4)