not only my father but also my mother is a teacher.还是not onl

not only my father but also my mother is a teacher.是正确的.
not only.but also这个短语采用“就近原则”,即谓语动词用单数还是复数,取决于离谓语动词最近的名词.这个句子中,离be动词（is）最近的是my mother, 为单数,所以be选择用is.
例如：not only you but also his parents like swimming. (离谓语动词like最近的是his parents, 谓语动词用单数形式like,而不用加s)

both意思为“两者都是”,即主语为复数,则谓语动词要用复数形式.
both my father and mother are teachers.

not only my father but also my mother is a teacher.
Both my father and my mother are teachers。