Pronouns are tripping people up. Consider the pronoun “I.” Standard usage dictates “I” should be used for the subject. It is now used frequently as the object as in, “My mother bought my brother and I new shoes.” You hear it everywhere, even from the mouths of those you would expect to know better.
A good way to check yourself is to move the preceding object from the sentence. You will catch the mistake immediately. Would you ever say, for example, “My mother bought I a new pair of shoes?” Of course you would not. I have even heard this usage from prominent television news commentators whose business requires using the language correctly.
Then there is that “he or she” business. When we tried to rid ourselves of sexist language, we stopped using the masculine “he” for everyone. Now we say “he or she” or “him or her,” as in “Anyone who wants to attend the conference should send his or her money by June 10.” That construction can get cumbersome and ridiculous. One simple way to fix this sentence is to change the subject to a plural. Change “anyone” to “people.” “People who want to attend the conference should send their money by June 10.”
However, the people have a simpler solution, especially in spoken English. Common usage is beginning to use the plural pronoun “their” or “them,” after a singular subject, as in “Anyone who wants to attend the conference should their money by June 10.”
The people speaking a language dictate how the language will evolve. These pronoun constructions are so widespread, our pronoun rules may change in a few hundred years. In the meantime, writers should know what they are doing. When they break the rules, they should do it for a reason and should know what they are doing. Otherwise, they are viewed as amateurs who don’t know better. A good writer knows how to use tools of the trade. (Note how I managed to move away from “his or her” before the word “tools”?) There is almost always a way to eliminate “he or she” and “his or her.”