prevent someone from + -ing
Stopping a person from doing something
Use from before an -ing form to show the action that is blocked.
The teacher prevented the students from cheating during the exam.
Prevent from or prevent by?
Prevent is commonly followed by from + activity or result. By usually introduces the method used to prevent something.
Quick answer
Which preposition is correct with prevent?
Use prevent from when someone or something stops an action or result: prevent someone from doing something. Prevent by is not the normal pattern for the blocked action, but by can introduce the method used to stop something.
The guard prevented visitors from entering the building.
We prevented flooding by installing better drains.
The law prevents companies from dumping waste here.
We prevented flooding by installing better drains.
The law prevents companies from dumping waste here.
Choose the pattern by meaning
These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.
prevent something from + noun/-ing
Stopping an event or result
Use from with events, damage, accidents, or changes.
This cover prevents the screen from scratches.
prevent something by + method
Showing the method of prevention
By introduces the action or technique used to stop a problem.
The company reduced accidents by improving training.
See the difference in context
The same subject can take different constructions because each sentence answers a different question.
Construction
Example
Meaning
prevent from
The vaccine prevents children from getting the disease.
Standard pattern for stopping an action or result.
prevent by
They prevented delays by adding more staff.
By introduces the method used to prevent something.
prevent someone by doing something
She prevented confusion by giving clear instructions.
Natural when by explains how the prevention happened.
prevent to
They changed the design to prevent leaks.
Prevent is not normally followed directly by to.
Common mistakes
Some combinations appear in real text but do not mean what learners often assume.
Using prevent by for the blocked action
Use from before the action that is stopped.
✕ The fence prevented children by entering. ✓ The fence prevented children from entering.Using an infinitive after prevent
Prevent is usually followed by from + -ing, not to + verb.
✕ The rules prevent employees to smoke. ✓ The rules prevent employees from smoking.Omitting from in formal English
In careful English, include from before the -ing form.
✕ The alarm prevented us leaving early. ✓ The alarm prevented us from leaving early.Quick practice
Choose the expression that matches the intended meaning.
1. The lock prevented anyone ___ opening the door.
2. They prevented water damage ___ fixing the roof quickly.
3. The policy prevents staff ___ sharing passwords.
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Frequently asked questions
Can I say prevent by doing something?
Yes. By can introduce the method used to stop a problem: prevent accidents by improving safety.
Is prevent from always necessary?
In standard English, from is normally used before the blocked action: prevent someone from leaving.
Can prevent be followed by a noun?
Yes. You can say prevent disease, prevent delays, or prevent damage without another verb.
Related preposition guides
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