back and forth   (前后)来回地，往返
back up 支持；倒退
be bent on (upon) 一心想做(某事)
be bound to 注定；决心
be able to 能，会
be about to 正要，即将(做)
be absent from 不在；缺席
be absorbed in 专心致力于…
be composed of 由…组成
be concerned with 关于…，与…有关；
be determined to 坚定；坚决；决心
be fit for 适合
be inclined to 倾向于…；想要
be obliged to (do) 被迫，不得不
be obliged to sb 感谢
relate .to 与 联系，理解
be related to 与…相关的，有亲戚关系
be/get tired/sick of 对…失去兴趣；厌烦
be acquainted with 开始认识；开始了解