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Prepositions after verbs

Strengthen by or strengthen in?

Strengthen is commonly followed by by for method or cause, and in for the area or quality that becomes stronger.

Quick answer

Which preposition goes with strengthen?

Use strengthen by to describe the means or action that makes something stronger. Use strengthen in to describe the area, skill, or quality that improves. The verb can also appear without a preposition.

The walls were strengthened by steel supports.
She has strengthened in confidence over the year.
The company hopes to strengthen its position in Asia.

Choose the pattern by meaning

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

1
strengthen by + method

Showing what causes improvement

By introduces the action, tool, or factor that makes something stronger.

The bridge was strengthened by additional cables.
Their case was strengthened by new evidence.
2
strengthen in + area

Showing the area of improvement

In refers to the quality, subject, or ability that becomes stronger.

He strengthened in leadership skills.
The team has strengthened in defence this season.
3
strengthen + object

Using strengthen without a preposition

The verb often takes a direct object with no following preposition.

The reforms will strengthen the economy.
Daily practice can strengthen your memory.

See the difference in context

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

Construction
Example
Meaning
strengthen by
The argument was strengthened by recent data.
shows the means or cause of strengthening
strengthen in
She strengthened in patience and discipline.
shows the area or quality that improved
strengthen with
They strengthened the roof with metal beams.
focuses on the material or tool added

Common mistakes

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

Using in for a method

Use by for the cause or method of strengthening.

✕ The proposal was strengthened in new evidence. ✓ The proposal was strengthened by new evidence.

Adding an unnecessary preposition

Strengthen often takes a direct object without a preposition.

✕ Exercise strengthens for the muscles. ✓ Exercise strengthens the muscles.

Quick practice

Choose the expression that matches the intended meaning.

1. The walls were strengthened ___ concrete supports.
2. She has strengthened ___ communication skills.
3. Regular reading can strengthen ___ vocabulary.

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Frequently asked questions

Can strengthen be used without a preposition?

Yes. It often takes a direct object, such as strengthen the relationship or strengthen the economy.

Is strengthen by common in passive sentences?

Yes. Passive structures like strengthened by evidence or strengthened by support are very common.

When do we use strengthen in?

Use strengthen in before a quality, field, or skill that becomes stronger.

sack, by, sack, for, sack, from "sacrifice for" or "sacrifice to"? sail from, in, for, to or through? "salvage from" or "salvage in"? sanction by, for, in, with or according? satisfy with, by, in, about or after? "saturate with" or "saturate in"? save by, for, from, on or in? "say in", "say about" or "say to"? "scan for", "scan in" or "scan through"? "scar by" or "scar on"? scatter in, across, around, throughout or on?

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