A complete sentence has a subject, a verb and a finished thought. When one of these is missing, or two thoughts are jammed together, the marker has to guess what you meant.
How do you check a sentence?
Ask three questions in order.
- Who or what is doing something? That is the subject.
- What are they doing or being? That is the verb.
- Could someone read this sentence alone and understand it? That is the finished thought.
If any answer is missing, you have a fragment. If you find two separate finished thoughts in one sentence with no proper joint, you have a run-on.
Worked example
Take this original Geography answer about a coastal town.
Because the town is near the sea. Many people work as fishermen and the market is busy every morning and the fish is sold fresh and tourists come to see it.
The first line fails the test. It has a subject (the town) and a verb (is), but the word because makes the reader wait for a result that never arrives. The second line holds four thoughts joined by and.
A repaired version follows.
The town is near the sea, so many people work as fishermen. The market is busy every morning and sells fresh fish. Tourists come to see it.
The repair joined the fragment to its result with so, then split the run-on at each new thought.
What does the common mistake look like?
A student starts an answer with Because or Which and puts a full stop before finishing the idea. For example: “The factory moved inland. Which reduced the cost of land.” The second part cannot stand alone.
The correct approach is to join it to the first: “The factory moved inland, which reduced the cost of land.” The comma and the joined thought make it complete.
Steps to follow
- Reread each sentence aloud and listen for where your voice would stop.
- Put a full stop only where the thought is complete.
- If a sentence has more than two and connectors, split it.
Self-check
Decide which of these original lines is complete, which is a fragment and which is a run-on. Then fix the broken ones.
(a) The volunteers cleaned the beach. (b) After the festival ended and the crowd left. (c) The tide came in the volunteers moved the bags the truck arrived at noon.
Answer
(a) is complete: subject volunteers, verb cleaned, finished thought. (b) is a fragment: After opens a clause that promises a result.
Fix: “After the festival ended and the crowd left, the volunteers cleaned the beach.” (c) is a run-on with three thoughts. Fix: “The tide came in. The volunteers moved the bags, and the truck arrived at noon.”
Where to go next
With clear sentences, the next step is to group them, which is covered in organising one idea per paragraph. The English writing task checklist lets you test your own sentences. Students working on English often start here, and the writing foundations hub shows the full sequence.