Type the verb that agrees with the subject press Enter. Does everyone including Steve and Bill know (his their) play book?

Answer 1

It could be either depending on context. If the play book in question is for an all-male team, then "his" is better. If there's a possibility that "everyone" could include females, then "their" is better, wrong as it may seem to those Of A Certain Age.

The insistence upon not using "singular they" is relatively recent, dating only back to about the mid-19th century. Native English speakers have been using "they" and its forms as a singular pronoun for a person of indeterminate gender for centuries, and recently even formal grammarians have been relaxing the strictures on this.

If this is a homework question ... sorry. Try to "read" your teacher. A Social Justice Warrior type will probably say "their" is the correct answer; the sort of person who says you should never split infinitives (true in Latin, not so much in English) will almost certainly say it should be "his".