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Prepositions after verbs

monitor by or monitor for

Monitor for is common when checking for signs or problems. By usually introduces method or agent, not the monitored target.

Quick answer

How do you use monitor with prepositions?

Use monitor for when watching for signs, changes, or risks: monitor for errors. Monitor by is less common and usually introduces a method: monitored by camera.

Doctors monitored the patient for infection.
The entrance is monitored by security staff.

Choose the pattern by meaning

These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.

1
monitor for + problem

Watching for signs or risks

For introduces the thing you are checking for.

Scientists monitor rivers for pollution.
2
monitored by + method/person

Showing who or what does the monitoring

By introduces the agent or tool in passive structures.

The area is monitored by satellite.
3
monitor + object

Using a direct object

Monitor often takes a direct object with no preposition.

We need to monitor customer feedback daily.

See the difference in context

The same subject can take different constructions because each sentence answers a different question.

Construction
Example
Meaning
monitor for
Nurses monitored him for fever.
Checking for a condition or sign.
monitor by
Traffic is monitored by sensors.
By introduces the monitoring method or agent.
monitor with
The team monitored air quality with portable devices.
With shows the tool used.

Common mistakes

Some combinations appear in real text but do not mean what learners often assume.

Using by for the target

By does not normally introduce the thing being checked.

✕ They monitored by errors. ✓ They monitored for errors.

Omitting the object

Monitor usually needs something being observed.

✕ Staff monitored for leaks. ✓ Staff monitored the pipes for leaks.

Quick practice

Choose the expression that matches the intended meaning.

1. The lab monitors samples ___ contamination.
2. The building is monitored ___ cameras.
3. We monitor customer activity every day. Which phrase is correct?

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Frequently asked questions

Is monitor for standard English?

Yes. It is common when checking for signs, conditions, or problems.

Can I say monitored by cameras?

Yes. By introduces the monitoring method or agent.

Does monitor always need for?

No. The verb often takes a direct object without a preposition.

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