助動詞の後ろは動詞の原形

　I      am very busy today.
　I will be very busy tomorrow.

                    of 〜                〜の
                    of the tennis club 　テニス部の
           a member of the tennis club 　テニス部の部員
 I      am a member of the tennis club.　私はテニス部員です。
 I will be a member of the tennis club.

　            私はテニス部員になるでしょう。