HSPT: Sentence boundaries: count the complete thoughts, then check the joint
Learn to tell a complete sentence from a fragment or a run-on, and to pick the punctuation that correctly joins two complete thoughts.
How can you tell whether a sentence is a fragment, a run-on, or joined correctly?
Short answer
Count the complete thoughts first. A word group with none is a fragment, and two complete thoughts need a period, a semicolon, or a comma plus a joining word such as and, but or so.
Know first: Subjects and verbs, Words like because, when and although
Step 1 of 6
A sentence needs at least one complete thought
A complete thought, also called an independent clause, has a subject and a verb and can stand alone: The ferry left at noon.
A word group can fall short in two ways. A dependent clause starts with a word like because, when, although, if, since or while, so it leaves you waiting for more: When the ferry left at noon. A phrase may have no main verb at all: The ferry, crowded with tourists. Either one written alone is a fragment.
| Word group | Complete? | Why |
|---|---|---|
| The ferry left at noon. | Yes | Subject ferry, verb left |
| When the ferry left at noon. | No | When makes it dependent |
| The ferry leaving at noon. | No | An -ing word alone can't be the main verb |
| The ferry, which left at noon. | No | The which part describes the ferry, but the ferry never gets its own verb |
| The ferry, which left at noon, was full. | Yes | Main verb was |
Cover any describing part between commas. If what's left can stand alone, it's a sentence.
Step 2 of 6
Two complete thoughts need a strong joint
When one sentence holds two complete thoughts, look hard at the spot where they meet. Some joints are strong enough to hold them, and two common ones are not.
| Joint | Example | Verdict |
|---|---|---|
| Period | The ferry left. We waited an hour. | Correct |
| Semicolon | The ferry left; we waited an hour. | Correct |
| Comma + and, but, or, nor, for, so, yet | The ferry left, so we waited an hour. | Correct |
| Make one part dependent | Since the ferry left, we waited an hour. | Correct |
| Comma alone | The ferry left, we waited an hour. | Comma splice |
| Nothing at all | The ferry left we waited an hour. | Run-on |
Step 3 of 6
Find the thoughts, then judge the joint
Worked example: A sentence with therefore
Which version is punctuated correctly?
- AThe power went out, therefore, we lit candles.
- BThe power went out; therefore, we lit candles.
- CThe power went out therefore we lit candles.
- DBecause the power went out; we lit candles.
- 1
Find the complete thoughts
The power went out and we lit candles each have a subject and a verb. That's two complete thoughts.
- 2
Check the joint
Therefore is a transition, not a joining word. It needs a semicolon or a period in front of it.
- 3
Explain the wrong choices
The first choice is a comma splice with therefore tucked inside it. The third runs the two thoughts together with no punctuation. The fourth puts a semicolon after a dependent clause, which needs a comma instead.
Answer
B. The power went out; therefore, we lit candles.
Find the wrong step
A student fixes this run-on: The dog barked all night nobody in the house slept.
- 1
Finds two complete thoughts: The dog barked all night and nobody in the house slept.
- 2
Adds a comma between them: The dog barked all night, nobody in the house slept.
What went wrong
A comma by itself can't hold two complete thoughts. The run-on is now a comma splice.
- 3
Decides the sentence is fixed.
The fix
Add a joining word with the comma: The dog barked all night, so nobody in the house slept. A period or a semicolon would also work.
Step 4 of 6
Test the boundaries yourself
Check yourself · Question 1
Which option is a complete sentence?
Answer: C
The volunteers carried boxes has a subject and a main verb and needs nothing more. Each of the others leaves you waiting for the rest of the sentence.
Check yourself · Question 2
Which option avoids both a run-on and a fragment?
Answer: C
The after clause is dependent, and the comma correctly connects it to the complete thought she printed two copies.
Trap answer: Maya finished her essay. Then printed two copies.
A period that creates a fragment
- Why it looks right
- It has a period between the two ideas, so it looks like the run-on was fixed.
- Why it's wrong
- The second word group lost its subject. Then printed two copies doesn't say who printed, so it can't stand alone.
- The right answer
- The version that starts with After keeps both ideas in one correct sentence.
Check yourself · Question 3
Which sentence uses however correctly?
Answer: B
Two complete thoughts meet at however, so a semicolon goes before it and a comma after it.
Common mistake
Thinking a run-on is just a sentence that goes on too long.
- Why it's tempting
- The name makes it sound like a length problem, so short sentences seem safe and long ones seem suspicious.
- Do this instead
- Length is never the test. I slipped I fell is a run-on with four words, and a long sentence can be perfectly correct. Count the complete thoughts and check what joins them.
Step 5 of 6
What to remember
Remember
- Count the complete thoughts before you judge any punctuation.
- Join two complete thoughts with a period, a semicolon, or a comma plus and, but, or, nor, for, so or yet.
- However and therefore need a semicolon or period before them, and a dependent clause alone is a fragment.
Step 6 of 6
Practice on a real question
Use what you just learned on this question, then check the explanation.
In your own words
In the practice question, which choice used only a comma between two complete thoughts, and what made the correct choice work?