①It seems amazing that at my age I am still fit enough to cycle 20 kilometers in an afternoon.
②It's my birthday in two weeks' time and I'll be 82 years old!
③Your mother tells me that you started smoking some time ago and now you are finding it difficult to give it up.
④I know how easy it is to begin smoking and how tough it is to stop.
⑤Get help if you need it!
探究发现
(1)it用法广泛，可以是代词，用来表示时间、天气、距离等，如例句②中it指时间，例句③中的第二个it指代smoking，例句⑤中it指代help。
(2)it可以作形式主语或宾语，如例句①中it作形式主语，真正的主语是后边的that从句。例句③中第一个it作形式宾语，真正的宾语是后边的动词不定式。例句④中的两个it都作形式主语。
