be back to do sth 中back 是什么词性?she will be back to parpre for dinner.

be back to do sth 中back 是什么词性?she will be back to parpre for dinner. be sure to do sth. be pleased to do sth be able to do sth be going to do sth be let to do sth? be sad to do sth. be made to do sth be obliged to do sth. be used to do sth. A let B do sth变为被动语态,是B be let do sth还是B be let to do sth be to do sth和will do sth的区别 be worth to do sth 还是be worth doing sth be scared to do sth还是be scared doing sth be surprised at sth.和be surprised to do sth. be glad to do sth 还是be glad doing sth be heard to do sth 还是 be heard doing sth? go back to do sth 和 go back doing sth 求区别