介词后面何时接定冠词?何时接不定冠词?

用the还是a还是不用与下列因素有关：
1）含义
Life on a desert island is wretched.荒漠岛上的生活很悲惨.意思是：任何荒漠岛都如此.荒漠岛是个变量.
Life on the desert island Captain Cook discovered was enjoyable.库克船长所发现的那个荒漠岛上生活很惬意.这句指的是库克船长所发现的那个荒漠岛,是个固定值.定冠词的用法之一.
fish live in water 鱼生活在水中,water不可数,水在这句中是变量,既可是河水,也可是湖水,也可是海水.
Fish cannot live in the water any more,because it is polluted.鱼已不能再这水里边生存了,因为它已被污染.这水指的是谈话人都知道的特定水.
2）固定说法 in time of,ahead of time,等：没有the,a
in front of 与in the front of； in hospital,in the hospital; go to school,
go to the school,; in bed,in the bed：有无冠词,意思有别,这是习惯.
3）含介词的地点状语有the的多,大多指说话双方都明白的特指对象.
fall to the ground; in the sky