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Prepositions after adjectives

Thorough in vs. thorough with

Use thorough in for areas of knowledge or skill and thorough with for careful handling or completion of tasks.

Quick answer

How these patterns differ

Thorough in describes expertise or completeness in a subject or activity. Thorough with describes careful attention to tasks, details, or people.

She is thorough in her research.
He's thorough with paperwork.
The lawyer was thorough in questioning witnesses.

Choose the pattern by meaning

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

1
thorough in + activity/subject

Showing care or completeness in an area

Use thorough in for detailed work or knowledge.

The team was thorough in its investigation.
2
thorough with + task/object

Handling something carefully

Use thorough with when referring to tasks, documents, or treatment of people.

She is thorough with financial records.
3
thorough with + person

Careful or strict treatment of someone

This use can suggest close attention or strictness.

The coach was thorough with the new players.

See the difference in context

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

Construction
Example
Meaning
thorough in
He's thorough in his work.
careful and complete in an activity or area
thorough with
She's thorough with reports.
carefully dealing with something
thorough about
She's thorough about safety checks.
also possible in some contexts, focusing on a topic or procedure
thorough on
The guide was thorough on legal issues.
less common; used with topics in some varieties

Common mistakes

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

Using in before objects or documents

Use with for tasks, paperwork, and items handled directly.

✕ She is thorough in reports. ✓ She is thorough with reports.

Using with for fields of study

Use in for areas of work or expertise.

✕ He's thorough with research methods. ✓ He's thorough in research methods.

Confusing thorough with thoroughly

Thorough is an adjective, while thoroughly is an adverb.

✕ She checked the files thorough. ✓ She checked the files thoroughly.

Quick practice

Choose the expression that matches the intended meaning.

1. The auditor was thorough ___ the investigation.
2. She's very thorough ___ contracts.
3. The professor is thorough ___ his explanations.

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Frequently asked questions

What is the main difference between thorough in and thorough with?

In refers to areas of activity or expertise, while with refers to handling tasks or objects carefully.

Can both forms appear in one sentence?

Yes. For example: She is thorough in her research and thorough with documentation.

Is thorough with people negative?

Not necessarily. It can mean careful, detailed, or sometimes strict.

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