推荐回答（5个）
答：there are aliens 是省略that的主语从句。
解析：
The base（主语）tries to find out（谓语）【if（if引导宾语从句，翻译为“是否”）it's possible（主系表，it为形式主语） there are aliens（省略that的主语从句，是it所指代的真正主语，关键主语是aliens） in the universe（地点状语）
翻译：基地致力于找出宇宙中是否存在外星人。
希望对您有帮助；）
在句子中担当主语的从句就叫做主语从句，通常放在主句谓语动词之前或由形式主语it代替，而本身放在句子末尾。通常结构为“引导词 + 主语 + 谓语 + 其他成分"。
二. 引导词：
1. that引导
eg. That you are coming to London is the best news I have heard this long time.
2. whether引导
eg. Whether they would come to conference was a question.
3. 连接代词引导: who,whom, whose, what, which, whoever, whomever, whichever, whatever
eg. Whoever is top from wins the game when two matched players meet.
4. 连接副词引导: therefore, besides, otherwise, however, moreover, still, thus, meanwhile
eg. How this happended is not clear to anyone.
5. 关系代词型what引导
eg. What shoked me was that she never wash her own clothes.
三. 形式主语it:
1. 对关系代词型what引导的主语从句，可以使用形式主语。
eg. It is clear enough what he meant.
2. 如果句子是疑问句，则必须用带形式主语it的结构.
eg. Is it true that all of us need to to conference?
好了以上就是这次为大家带来的关于主语从句的知识点讲解，大家都学得怎么样呢？千万不要忘记之后的复习哦！
基地试图查明宇宙中是否有外星人。
The base主 tries谓 to find out if it's possible there are aliens in the universe.宾
宾语从句：if引导词 it主's系 possible表 (that省略了） there are aliens in the universe（that从句是真正主语）.
这里不是省略了that，而是这里压根就没有tnat。it's ossible there are aliens in the universe是不定式短语 to find out的宾语从句，但它的引导词是 if （是否），if倒是可以换成whether 更正式一些。
英语句子成分主语从句省略that？
the base tries to find out if it‘s possible there are aliens in the universe 这里是省略了that嘛
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