When we write about a person, it is sometimes not clear whether we refer to a man or a woman. Words such as somebody or person are neutral and can refer to either gender. That causes a problem for instance when we need to use a pronoun in the singular and still want to be gender-neutral.
One way is to write he or she and his or her or he/she and his/her.
Somebody left his or her umbrella on the train.
The preferences a person has about what he/she does should be taken into account.
However, writing he or she, etc., looks a bit clumsy. Using they even if we talk about one person is nowadays generally accepted also by most style guides.
Somebody left their umbrella on the train.
Why would anybody want to end their life?
Each child played with their own toys.
Every teacher used their own method.
In fact, they has been used in the singular (in writing) since the 14th century.
We can use the singular form themself if we refer to one person (and themselves is also correct).
Everybody must look after themself (or themselves).
It’s all about letting someone be themself. (Cambridge Dictionary)
To language purists the singular form themself may seem unnatural. However, it was used in English as early as the 1300s and there are examples from Emily Dickinson and F. Scott Fitzgerald well over a hundred years ago. Even if themself is still seen by a majority as nonstandard, the word is gaining popularity. And it is practical. After all, we write yourself and yourselves.
They is nowadays sometimes used to refer to a person whose gender is nonbinary, that is, who wants to be identified as neither male nor female. This use may still sound strange to many people.
Kim, our new coworker, wants to be referred to as they.
Sam drinks their coffee without sugar.
They was Merriam-Webster’s Word of the Year 2019.
Traditionallly, he used to refer to a person whose gender was unknown, but that use has come to be seen as sexist. Now some writers seem to want to counteract male dominance by using only the female forms she and her even when the reference may be to a neutral word such as person.
How can a person make sure that her views are taken into consideration?
To sum up, use they (and them, their) when the gender of the person referred to is unknown or irrelevant.
Avoid writing combined forms such as s/he or (s)he.
You can read about the use of gender-neutral titles here.