281.I wonder what happened to him.（我不知道他做了什么事。）讲解

281.I wonder what happened to him.（我不知道他做了什么事。）

280.I won't buy your story.（我不信你那一套。）

280.I won't buy your story.（我不信你那一套。）讲解

279.I'm behind in my work.（我工作进度落后了。）

279.I'm behind in my work.（我工作进度落后了。）讲解

278.I'd like to try on this hat.（我想试试这些帽子。）

278.I'd like to try on this hat.（我想试试这些帽子。）讲解

277.If I had known that I could have helped you.（假如我早知道我就帮你了。）

277.If I had known that I could have helped you.（假如我早知道我就帮你了。）讲解