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Prepositions after verbs

modify by or modify at

Modify commonly takes by to show degree or method of change. Modify at is not a standard complement after the verb.

Quick answer

Which preposition is correct after modify?

Use modify by to show an amount, factor, or method of change: modified by 10%, modified by heat. Modify at is generally not used as a fixed pattern after the verb, though at may introduce a separate place or rate phrase.

The recipe was modified by a nutritionist.
Sales were modified by seasonal demand.
The settings were modified by 5 percent.

Choose the pattern by meaning

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

1
modify by + amount

Showing the degree of change

By introduces the quantity or percentage of change.

Prices were modified by 2% last month.
2
modify by + agent/method

Showing what caused the change

By can introduce the person, tool, or condition responsible for the modification.

The design was modified by software before printing.
3
modify + noun

Using a direct object

Modify often takes a direct object without a preposition.

They plan to modify the contract tomorrow.

See the difference in context

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

Construction
Example
Meaning
modify by
The dosage was modified by the doctor.
Standard pattern for cause, agent, or amount of change.
modify at
The machine was modified at the factory.
At refers to location, not a fixed verb-preposition pattern.
modify with
The sauce was modified with extra herbs.
With introduces an added ingredient or feature.

Common mistakes

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

Using at as the main complement

At usually does not follow modify directly to complete its meaning.

✕ They modified at the rules. ✓ They modified the rules.

Forgetting the object

Modify normally needs something that is changed.

✕ The engineer modified by 3%. ✓ The engineer modified the output by 3%.

Quick practice

Choose the expression that matches the intended meaning.

1. The settings were modified ___ 15%.
2. They modified the software ___ adding new filters.
3. The equipment was modified ___ the workshop.

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Frequently asked questions

Is modify at correct English?

Not usually as a verb-preposition combination. At is more often a location phrase.

Can I say modified by 10 percent?

Yes. This is a standard way to express the amount of change.

Does modify need an object?

Usually yes. You modify something such as a plan, system, or document.

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