There常规上来说是一个副词意思是在那地方，但有时也把它当代词使用，其所指代的通常是表示地方的名词：
◆ Wrong: At current prices, Antarctic oil may be worth drilling for, if wells can be dug THERE and environmental concerns addressed.
◆ Right: At current prices, oil in Antarctica may be worth drilling for, if wells can be dug THERE and environmental concerns addressed.
One 通常指的不确定的，非特指的单数词：
After walking by the chocolates so many times, Roger finally had to eat ONE.
上例中的chocolate 没有特定指哪一个，而如果用其他代词如it ,they 都有明确特指哪个chocolate ：
After walking by the chocolates so many times, Roger finally had to eat THEM.
在这个例子中，Roger 吃了所有的chocolate ，而如果我们特指的是单数就要用it :
After walking by the chocolates so many times, Roger finally had to eat ONE. IT was delicious, but HE could eat only half of IT.
