Undefined subject
When I have a sentence where the subject is undefined, which pronoun should I use: he, she or it?

Okay, in the case of objects certainly I should use “it”, but how about when the subject is alive, like a person, for example?

e.g. “Each one must have his/her own opinion about that.”

Thank you.

Question

Undefined subject
When I have a sentence where the subject is undefined, which pronoun should I use: he, she or it?

Okay, in the case of objects certainly I should use “it”, but how about when the subject is alive, like a person, for example?

e.g. “Each one must have his/her own opinion about that.”

Thank you.

Answer

The correct pronoun to use in these cases is "they, them, their".
In earlier times the standard was always to use the masculine pronoun, but that has gone out of fashion.

Each unto their own.
Everyone to their beds, now.
I want everybody to bring their books to class.

"His or her" is terribly pedantic. It may be politically correct, but no one gives a rat's backside for that really.

Answer

It is correct to say "his/her" if you are writing.  If you are speaking, you can say "his or her" as in, “Each one must have his or her own opinion about that.”

While this next one is not technically correct if you are specifically talking about one person, but it is common to hear 
"Each one must have their own opinion."  
Like I said this is technically not correct because you are talking about "each one" (singular) and "their" is plural, but it is very common to hear in spoken English conversation.

Otherwise, stick with "his or her"

لا تفوّت فرصة تعلّم لغة جديدة وأنت مرتاح في منزلك. تصفّح مجموعتنا المختارة من مدرّسي اللغات ذوي الخبرة وسجّل في درسك الأول الآن!