这个其实很好理解的。答案是加不加都可以，但是两者的情况稍有不同：
加介词时，there的词性是代词pronoun，指代一个具体的地方。
- Let's go find Susan in the shopping mall! She's been in there for 4 hours already!
- there = the shopping mall
- in there = in the shopping mall
不加介词时，there的词性是副词adverb。
- Let's go find Susan in the shopping mall! She's been there for 4 hours already!
意思一样的，只是词性不同。是不是很简单？哈哈~
该是at there吧？
没记得有in there放句末的情况啊……
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