It's like how you use causative(make, let, have) in English. The sentence(没有分开他们) still makes sense if you don't write the 給.
1 de abril de 2015
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the action 分开 doesn't come from "它们", we dont know where it comes from, maybe some people. In this case, we dont care who did this action, we just care the result of this action, so we add 给