1-The need for pronoun-antecedent agreement can create gender problems. If one were to write, for instance, "A student must see his counselor before the end of the semester," when there are female students about, nothing but grief will follow. One can pluralize, in this situation, to avoid the problem:
- Students must see their counselor before the end of the semester.
Or, one could say
- A student must see his or her counselor. . . .
Too many his's and her's eventually become annoying, however, and the reader becomes more aware of the writer trying to be conscious of good form than he or she is of the matter at hand.
2-Trying to conform to the above rule (#1) can lead to a great deal of nonsense. It is widely regarded as being correct (or correct enough), to say
- Somebody has left their bag on the floor.
but many people would object its being written that way because somebody is singular and their is plural. There is a great deal to be said, however, for using the word their as the gender-non-specific, singular pronoun.
in short , ---body or ---one takes plural pronoun ( they, them, themselves etc.) ---thing takes the pronoun "it" ( its, itself etc.)