They haven't decided yet which hotel to stay at.Animals will commonly use force,or a show of force,to decide which will stay.这两句中stay后面 为什么一个要接上at,另一个不用呢?

第一个是which引导的定语从句，相当于 to stay at which hotel,第二个只看意思，意为留下，存活

They haven't decided yet which hotel to stay at. 他们还没有决定要入住哪家旅店。这里的stay后面有宾语which hotel。stay这个词后面不能直接跟宾语，所以要加介词at。
Animals will commonly use force to decide which will stay.动物们将使用武力来决定哪一个留下。这里的stay后面没有宾语所以不用加at.

They haven't decided yet which hotel to stay at.
这一句which hotel to stay at
=which hotel they should stay at
=they should stay at which hotel
stay后接at才能接宾语的.
Animals will commonly use force,or a show of force,to decide which will stay.
这一句which是充当will stay的主语.所以,stay后面不接宾语,也就是说,不用加at

第一个是说呆在哪个宾馆，第二个是说谁会留下来