he/she uses or they use?

[itecompro]

Hi.

What is the usage rule of "he/she" when we don't know the sex of the person?

he or she uses / they uses / they use?
his or her book / their book / their books?

which one is correct for writing and which one for speaking?

Thanks.

 

[lsah3]

Both are correct. I personally find the first one too long, and thus use they/their more often.

EDIT: Of course I mean the first two forms in both examples.

 

[Barb_D]

They uses is incorrect.

Book or books depends on the number of books not the number of owners.