Another English question!

Well, all the students above got it right, but I can add a few points.

1. What a nice day!

Weather is an uncountable noun, which means "weather" and means "what a nice day!" . This sentence is also equal to: How nice the weather is!

The sentence pattern of the former is: what+ adjective+noun (+subject+predicate verb)+!

What fine weather! What fine weather!

The latter sentence pattern: how+ adjective (+noun) (+subject+predicate verb)+!

It's really cool today!

Day is a countable noun, so an A must be added before the noun to indicate the number.

This sentence is equal to: What a cool day it is today!