why, how, who, what should sb/ sth do used to refuse sth or to show that you are annoyed at a request; used to express surprise about an event or a situation （表示拒绝、恼怒或惊奇） ： Why should I help him? He's never done anything for me. 我干吗要帮他呢？他从来没为我做过什么。 ◊ How should I know where you've left your bag? 我怎么知道你把包丢在哪儿了？ ◊ I got on the bus and who should be sitting in front of me but Tony! 我上了公共汽车，没想到坐在我前面的竟然是托尼！

Should

【a】 (used with question words to express lack of interest, disbelief, etc 与疑问词连用, 表示不感兴趣、 难以相信等): How should I know? 我怎么知道呢? Why should he think that? 他怎么那么想呢?

【b】(used with question words to express surprise 与疑问词连用表示惊讶): I was thinking of going to see John when who should appear but John himself. 我还想去看约翰, 想不到约翰来了. I turned round on the bus and who should be sitting behind me but my ex-wife. 我在公共汽车上转过身来, 谁料后面坐的竟是我的前妻.

emphasizing how surprising an event was ［强调事件的惊讶］竟然会 I was in this shop when who should I see across the street but Tobias.我在这家商店时，竟然看见托比阿在街对面。

should (SURPRISE) used to express surprise in sentences that are in the form of questions: I was just getting off the bus when who should I see but my old school friend Pat!

what should I see but something/who should appear but somebody etc used to show that you were surprised when you saw a particular thing, when a particular person appeared etc你猜我看到了什么/你猜谁来了（用于表示惊讶）: Just at that moment, who should walk in but old Jim himself. 就在那里，你猜猜谁进来了？就是老吉姆本人。