【问】 It seemed that the headmaster would come to our class.请问that 从句是什么从句?
 【答】 有的语法学家(例如《英语语法札记》的作者薄冰教授等)认为that从句是主语从句，是全句的真实主语，而it是它的形式主语。但这种解释会遇到一个无法解决的困难：主语从句可以提前，而这里的that从句却是无论如何也不能提前的：
 ×That the headmaster would come to our class seemed.
 比较：
 It seemed strange to my brother that I liked physics.→That I liked physics seemed strangeto my brother.
 这说明seem之后必须跟有主语补语(表语)，才能把that从句放到句首，否则句子便不能成立。
 因此，对于你所问的句子我们必须作另外的分析：此类句型中的it是无人称it(imper-sonal it)或虚设it(expletive it)，充当句子的主语。它只是填补句法上的空缺，并无实际含义。seem在这里是联系动词，要求有主语补语(表语);that从句便是它的主语补语。从信息上看，主语补语总是表示新信息，具有较大的信息载荷：
 The blue dress is oldest.
 这兰色的衣服很旧。
 The oldest dress is blue.
 这很旧的衣服是兰色的。
 而在It seems that句型中，that从句总是新信息的载体：
 1. It seemed that everyone was satisfied.
 看来每个人都很满意。
 2. It seems (that) she was married and recently divorced.
 看来她是结了婚，最近又离婚了。
 3. It seems that he was late for the train.
 他似乎没赶上那班火车。
 4. It happened that the weather was exceptionally cold.
 碰巧天气非常冷。
 5. It appears that he followed my advice. (EED)
 看来他听取了我的劝告。
 6. It happened that I was not in London at the time. (EED)
 碰巧那时我不在伦敦。