it seems to be the only choice left 句式分析

When you show up,it seems only you and me left in the whole world.\u662f\u795e\u9a6c\u610f\u601d\uff1f

\u5f53\u4f60\u51fa\u73b0\u65f6\uff0c\u6574\u4e2a\u4e16\u754c\u4f3c\u4e4e\u53ea\u5269\u4e0b\u4e86\u4f60\u548c\u6211\u3002

\u5f53\u4f60\u51fa\u73b0\u7684\u65f6\u5019\uff0c\u597d\u50cf\u6574\u4e2a\u4e16\u754c\u90fd\u53ea\u5269\u4e0b\u6211\u548c\u4f60~~~

it 是形式主语（不定式太长丢在后面更地道，不然放句首有头重脚轻之感）
seems是谓语（时态是一般现在时）
to be the only choice left是不定式充当真正的主语
the only 是表语（修饰choice，表限定）
left是宾语补足语（用过去分词修饰choice，表示剩下的状态）

句型：It seems/seemed to be ...
形式主语：it
谓语部分：seems
真实主语：to be the only choice left
其中，left是过去分词，作为后置定语修饰choice

翻译：这似乎是被留下的唯一的选择。

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it是主语
seems to be 是系动词
the only choice 是表语
left是定语
或
it 是形式主语
seems是谓语
to be the only choice left是不定式短语 作真主语

这似乎是唯一的选择。
这就简单地主谓宾宾补结构，left shi 过去分词后置修饰choice。
在这里it是带刺，并非作形式主语。

扩展阅读：we saw the houses of the ... seems to be doing ... it seems to me that ... it seems to be a law in ... it seems adj to do ... it strikes sb that ... there seems to be ... some kind of ... prove to do sth ...