When do you suppose they'll be back? 张道真教授编著的《实用英语语法》(外研社，p.612)认为上句中的do you suppose是插入语。但我觉得它跟一般的插入语不同。请分析。
张道真教授的分析方法代表了一派英语语法学家的观点。但是，仔细分析起来，上句中的 do you suppose 并不符合插入语的定义，因为去掉它之后，句子就不完整了：
When do you suppose they'll be back? →When they'll be back? (×)
因此,另一派英语学家提出了另外的分析方法:do you suppose 是混合疑问句的一个组成部分,见 interrogative sentences, suggest, think, suppose, believe。
此类句型我国英语教科书中曾多次出现：
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What do you think should be done to people who are caught stealing? 发现有人正在偷东西的时候,你认为应当如何处置?
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What do you think I should do then? 那你认为我应当怎么做呢？
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Which food do you think is healthy and which is unhealthy? 你认为哪种食物是健康食物,哪种是不健康食物呢?
全国高考也多次出现此类试题,考生应当认清它的结构本质:
- Which do you think tastes ____, the chicken or the fish?
A. well B. good C. better D. best(全国高考)
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— I haven't heard from Henry for a long time. — What do you suppose ____ to him? A. was happening B. to happen C. has happened D. had happened(上海高考,97-11)
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____ you have seen both fighters, ____ will win? A. Since ... do you think who B. As ... who you think C. When ... whoever D. Since ... who do you think(同上,95-19)
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What do you think made Mary so upset? ____ her new bicycle. A. As she lost B. Lost C. Losing D. Because of losing(同上,97-13)