What is the error in this sentence: "Several lost his or her keys on this train."?

The sentence "Several lost his or her keys on this train." contains a pronoun agreement error. The subject "several" is plural, which means that it requires a plural pronoun to align with it. The phrase "his or her" is singular, which creates a mismatch between the subject and pronoun.

In English grammar, when using a plural noun (like "several"), the corresponding pronoun should also be plural to ensure both agreement in number and clarity. A more appropriate phrasing in this context would be "their" to refer back to "several" without losing the plural sense.

For example, the corrected version could read, "Several lost their keys on this train." This maintains grammatical accuracy and effectively conveys the intended meaning.