Underline usually takes a direct object. By and in are used for method or location, not as fixed verb-preposition combinations.
Use underline with a direct object: underline a word, underline the importance. By can describe means, and in can describe location or medium.
These forms are not interchangeable. Choose the construction by meaning, not by which preposition appears most often.
Underline commonly takes a direct object without a preposition.
By introduces the action used to emphasise something.
In can describe colour, style, or the document where something is underlined.
The same subject can take different constructions because each sentence answers a different question.
Some combinations appear in real text but do not mean what learners often assume.
Underline directly takes the thing being marked or emphasised.
✕ Underline on the answer. ✓ Underline the answer.By should introduce an action or means.
✕ She underlined by the problem. ✓ She underlined the problem.Choose the expression that matches the intended meaning.
Paste the complete sentence below. Linguix checks the preposition together with grammar, spelling, punctuation, and style.
Yes. It often means emphasise, as in underline the importance of teamwork.
No. By usually introduces the method used to create emphasis.
In describes the colour or style used for the marking.
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