When I was on the train yesterday, I heard someone say this: "Someone left their bag on the train."

Can you find anything wrong with the sentence?

If you looked in a traditional English grammar book, you would learn that the sentence should be, "Someone left his bag on the train." The rule is to use the singular pronoun "he" when the gender of a person is not known.

But, if you asked native English speakers if there is something wrong with the sentence, many would probably answer no.

The pronoun "their" is generally plural. The speaker was talking about just one person. However, American English speakers use "their" and "they" as singular pronouns all the time in spoken English. They use it when the gender of a person is not known. They also use it when they do not want to say the gender.

Writers also try to use both "he" and "she" to show they do not discriminate against females. They might also use "he/she" or "him/her" instead of choosing one singular pronoun.

Another approach to this problem is to use a gender-neutral pronoun - a word that does not show gender. One place where these pronouns became popular is in virtual or online communities.

In Sweden, two nursery schools have used the gender-neutral pronoun, "hen" since 2012. The Swedish government started using "hen" this year and added it to the official dictionary.

A student organization at the University of Wisconsin recommends using gender-neutral pronouns like those in the following chart to respect transgender individuals.

Teresa Schmedding is an editor at the Daily Herald Media Group and a member of the American Copy Editors Society (ACES.)

At a recent meeting of the organization in Pittsburgh, she says some editors discussed the use of the pronoun "they." Ms. Schmedding says some members were unhappy with the use of "his" or "her" in the stories.

"Language is a constantly evolving thing and we need to evolve. ... It has become so common, in our language now, that people frequently use the singular ‘they' all the time. My question is, what's the harm?"

Writers look to books like "The Chicago Manual of Style" for the rules. This book says to use a plural noun, if possible, and to avoid using the singular pronouns "him" or "her" when the gender of the subject is not stated.

For example, the sentence

"Each student brought his or her book to class"

would change to

"The students brought their books to class."

Ms. Schmedding says the most important thing for writers is to make the language easy to understand.

"The overriding issue is clarity. We want people to understand what we are saying. So if people already understand when you use the singular they what you mean, why make up a new word?"

A look back at the history of English shows that great writers used they as a singular pronoun. Chaucer, writing in the 14th century, used it, as did Shakespeare, Jane Austin, and George Bernard Shaw.

Then in the late 18th century, grammar writers said they should not be used as a singular pronoun.

Today, many English speakers are saying that, "if everyone uses it, they must be right."

I'm Jonathan Evans.