1.seem to do sth中,seem后面加的是不定式的一般形式；
而seem to have done sth中,seem后面加的是不定式的完成形式.
后者表示不定式的动作发生在seem动作之前.
2.第一句变化中,seems为一般现在时,即seem发生在现在,而was here为一般过去时,发生在过去.
也就是说不定式to be here发生在seem之前,所以使用其完成形式,即变成了：seems to have been here；
而第二句变化中,seemed和knew都是用过去式,表明这两个动作同时发生,所以只需要用seem to do sth即可.
3.It seems to be + 表语 结构中,to be经常可以省略,尤其是加表语从句时,此时一般没有to be.
所以It seems后面可以加名词性从句,it seems to be后面不加.
其表语从句一般由that,as if或as though引导.
如：It seems that you are tired.看起来你似乎累了.
It seems as if/though it is going to rain.看起来似乎要下雨了.
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