it is a fine day today 近今天天气很好的意思 it is 是指它的意思为什么成天气了呢

it是个代词,可以做人称代词和非人称代词.
做人称代词时：
1,代替全面已提到过的事物,避免重复
例如：where is my book? it is on the desk.
2, 用于指动物或性别不详的婴儿,以及指代不明确的人.
例如：Look at the baby. it's my nephew.
The door bell is ringing. who is it?
3,用于指抽象事物.
例如：thank you for your help, i'll never forget it.
做非人称代词时：
1,it可以指天气、时间、地点、温度、距离等.
2,可以充当形式主语和形式宾语.
例如：it is important to study English.
3,强调
例如：It was my teacher who called my up.