表示喜怒哀乐等强烈感情的句子叫做感叹句,感叹句句末常常用感叹号,一般用降调,句首多用how和what,其余与陈述句结构相同.
how在感叹句中可以修饰形容词、副词、谓语动词等,如：
How beautiful it is!
How tall a boy he is!（how修饰形容词,这时形容词放在不定冠词a之前,注意,不能说How tall boys they are!要说What tall boys they are!）
How well the clothes fit!
How I missed you!
How wonderful it is!
How strange is his appearance!
what在感叹句里修饰名词,名词如果是单数的可数名词,把a放在what的后面,而不可数名词和复数名词之前,多用what.如：
What an opportunity it is!
What a tall boy he is!（what修饰名词短语a tall boy）
In what a mess his office is!
What fools they are!
What beautiful pictures they are!（what修饰名词短语beautiful pictures）
你给出的句子是错的,这是一个感叹句,要感叹的中心词是fine weather,由于weather是一个不可数名词,所以前面不能加不定冠词a,这种情况下,我们用what来修饰这个不可数名词,表示“这天气多么好啊!”希望我能帮助你解疑释惑.