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A compound sentence is a sentence formed from two or more simple sentences, or independent clauses (IC):
- Megan cut the wood, and Ryan stacked it.
Note that the two clauses forming the compound sentence must be related in meaning. Because of the logical connection between the two clauses in the example above, they fit together easily in one sentence.
Three methods of forming compound sentences
There are three ways of joining independent clauses into a compound sentence:
- with a coordinating conjunction (one of the fanboys);
- with a semicolon; or
- with a semicolon and a transitional expression.
Method 1: Use a comma and a coordinating conjunction (IC, cc IC)
You can use a comma and a coordinating conjunction (cc) to join two or more independent clauses into a compound sentence. (A coordinating conjunction is one of the fanboys: for, and, nor, but, or, yet and so.)
The resulting sentence has the structure IC, cc IC:
- For: The two stopped to eat, for the work had made them hungry.
- And: The afternoon had been long, and hours had gone by since lunch.
- Nor: There was no house nearby, nor did they have any food with them.
- But: They wanted to pick blueberries as a snack, but a bear growled at them from the berry patch.
- Or: Should they set off for home now, or should they decide to work a little longer?
- Yet: There was still work to do, yet they needed to rest and eat.
- So: They were starving and exhausted, so they went home.
Note that the comma in the above sentences comes before the conjunction, not after it.
If the independent clauses are very short, parallel and closely related, it is acceptable to use only a comma or a conjunction to join them:
- Megan cut the wood, Ryan stacked it.
- Megan cut the wood and Ryan stacked it.
But between longer clauses, it is important to use both a comma and a conjunction:
- Megan cut the wood into one-and-a-half-foot logs, and Ryan stacked them carefully along the rear of the cabin.
If you used a comma alone in the last example, you would have a comma splice, a type of run-on sentence. Since it is never wrong to use a comma and conjunction together in this kind of compound sentence, it is best to do so in every case to avoid the risk of error.
Method 2: Use a semicolon (IC; IC)
Like a period, a semicolon (;) creates a stop between two independent clauses. However, the clause after the semicolon does not begin with a capital. In this case, the sentence has the structure IC; IC.
This method works readily with some of the examples:
- The two stopped to eat; the work had made them hungry.
- The afternoon had been long; hours had gone by since lunch.
However, it would be awkward to use a semicolon by itself to form some compound sentences. The sentence below, for example, is confusing:
- Incorrect: There was still work to do; they needed to rest and eat.
These two clauses express contrasting ideas. We need a contrast word to make the connection clear. In this case, you might use the contrast conjunctions but or yet with a comma (Method 1), or you might decide to use Method 3.
Method 3: Use a semicolon with a transitional expression (IC; transition, IC)
A transitional expression is a word or phrase that shows the relationship between two ideas. A semicolon with a transitional expression often makes a smoother connection than a semicolon alone.
There are many transitional expressions showing different kinds of relationships. Here are a few common examples:
- to add a similar idea: also, in addition, moreover
- to show contrast: however, in contrast, on the other hand
- to indicate a result: therefore, as a result, consequently
- to introduce an example: for example, for instance
- to list points in order: first, second, third; next; then
Note that a comma is used after the transitional expression. The sentence has the structure IC; transition, IC:
- The work had made them hungry; therefore, they stopped to eat.
- The job was not done; on the other hand, they needed to rest and eat.
Keep in mind that a transitional expression is not a conjunction; it does not join sentences grammatically. Therefore, a period or a semicolon is needed before a transitional expression between two independent clauses:
- They wanted to pick blueberries as a snack. However, a bear growled at them from the berry patch.
- They wanted to pick blueberries as a snack; however, a bear growled at them from the berry patch.
If you use a comma instead of a period or a semicolon, you will have a comma splice run-on:
- Run-on: They wanted to pick blueberries as a snack, however, a bear growled at them from the berry patch.
Be careful to avoid this type of run-on; it is a very common error.
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