Compound Sentences

A compound sentence is made by joining two simple sentences together using a conjunction. Each part of the compound sentence can stand on its own as a complete thought.

What You'll Learn

How to join two simple sentences into one compound sentence using conjunctions
Which conjunctions to use: and, but, or, so
Where to place a comma when joining two sentences

When to Use

Adding information: "I like swimming, and my brother likes cycling."
Showing a difference: "She wanted to play outside, but it was raining."
Giving a choice: "We can go to the library, or we can visit the museum."
Showing a result: "The bus was late, so we walked to school."

How to Form

Step 1: Start with Two Simple Sentences

A simple sentence has one subject and one verb. It expresses one complete idea.

Simple Sentence 1	Simple Sentence 2
I finished my homework.	I went to the playground.
The hawker centre was crowded.	We found a table.
You can have chicken rice.	You can have laksa.

Step 2: Choose the Right Conjunction

Pick a conjunction that shows how the two ideas are connected.

Conjunction	What it does	Example
and	adds an idea	I finished my homework, and I went to the playground.
but	shows a difference	The hawker centre was crowded, but we found a table.
or	gives a choice	You can have chicken rice, or you can have laksa.
so	shows a result	It started to rain, so we ran inside.

Step 3: Add a Comma Before the Conjunction

When you join two complete sentences, put a comma right before the conjunction.

Pattern: Simple sentence , conjunction simple sentence.

Key Rules

Both parts must be complete sentences: Each side of the conjunction must have its own subject and verb. "I ran, and she walked." (correct) vs "I ran and walked." (this joins two verbs, not two sentences)
Use a comma before the conjunction: When joining two complete sentences, always place a comma before the conjunction. "The cat sat on the mat**,** and the dog lay on the rug."
Pick the right conjunction for the meaning: "And" adds, "but" contrasts, "or" offers a choice, "so" shows a result. Choosing the wrong one changes the meaning of your sentence.
Do not use a comma when joining only words or phrases: You only need a comma when both sides are complete sentences. "I like apples and oranges." (no comma needed -- joining two nouns, not two sentences)
A compound sentence is different from a simple sentence with two verbs: "She sang and danced." is a simple sentence (one subject, two verbs). "She sang, and he danced." is a compound sentence (two subjects, two verbs, two complete ideas).

Common Mistakes

Wrong	Right	Why
I was tired I went to bed.	I was tired, so I went to bed.	Two sentences need a conjunction to join them
The food was good, I ate a lot.	The food was good, so I ate a lot.	A comma alone cannot join two sentences -- you need a conjunction
He is tall, and strong.	He is tall and strong.	"Strong" is not a complete sentence, so no comma is needed
I wanted to swim so the pool was closed.	I wanted to swim, but the pool was closed.	"But" shows contrast; "so" shows a result (the meaning is wrong)
We went to the park, or we played football.	We went to the park, and we played football.	"Or" gives a choice -- if both things happened, use "and"

Clue Words

Conjunctions that join two sentences:

and, but, or, so

Words that signal addition:

and, also, too

Words that signal contrast:

but, however

Words that signal a choice:

or, either

Words that signal a result:

so, therefore

Tip: Remember the four conjunctions for compound sentences as ABOS -- And, But, Or, So. If you can split a sentence back into two complete sentences by removing the conjunction, it is a compound sentence!

Practice Tips

The split test: Cover the conjunction and read each side separately. If both sides make sense on their own, you have a compound sentence. If one side does not make sense alone, it is not a compound sentence.
Match the meaning: Before choosing a conjunction, ask yourself: "Am I adding, contrasting, giving a choice, or showing a result?" Then pick the conjunction that matches.
Comma check: After you write a compound sentence, look for the comma. It should come right before the conjunction. If you forgot it, add it in.
Combine short sentences: When you see two short sentences in a row about related ideas, try joining them with a conjunction to make your writing smoother.

Quick Reference

Conjunction	Meaning	Example Compound Sentence
and	adds another idea	I packed my bag, and I walked to school.
but	shows a difference	She studied hard, but the test was difficult.
or	offers a choice	We can take the MRT, or we can take the bus.
so	shows a result	The weather was hot, so we bought cold drinks.

Part	What it needs
Simple sentence 1	A subject + a verb (complete idea)
Comma	Placed right before the conjunction
Conjunction	and, but, or, so
Simple sentence 2	A subject + a verb (complete idea)

Compound Sentences (P3) (Primary 3)