how do you study for a test？答语中，by后为什么加ing？
how do you study for a test？ 回答的：by working friends为什么by后动词要加ing呢？ 答案的全句应该是：I study for a test by working friends. by 后动词加ing 是因为 by 介词，介词后跟的动词必须加ing. 对不？
how do you study for a test？ 回答的：by working friends为什么by后动词要加ing呢？ 答案的全句应该是：I study for a test by working friends. by 后动词加ing 是因为 by 介词，介词后跟的动词必须加ing. 对不？