it,one 和 that一向是一个备考的热点问题,前些年的试卷中经常出现,不过现在好像很少见到了.
it,只能是特指上文已经提到过的事物,或是双方都清楚的事物.如 Look at that car,I bought it last week.
它还有许多其他的用法,如表示说话人所指的某一种形势或情况.
If a fire breaks out in this market,it may cause a lot of deaths.
或是说话人所指的某一种情况如,I hate it when people talk with their mouthes full.
它还可以用来表示身份或性别不详之人,如 Somebody's knocking at the door.Who might be it?
它还可以用作形式宾语和形式主语,还可以用在it is .that/who句型中表示强调,还可以用来表示天气、距离等.
one的意思为某一个,多指与上文所提事物同一种类的不确定的某一个.
I lost my pen,can you lend me one?
There are two apples in the basket.One is green,while the other one is red.
that多指上文所提事物的同类,但是这些事物大多是一些抽象的概念.注意这里的that并不是特指哦.
如您的题目中那个that实际上指的是伦敦的天气,和上文中北京的天气属同一类别.还有一个经典的例句：
The population of China is larger than that of Japan.