「how」と「what」の違いは何?

The other day I read this:

× How fantastic party it is!!
○ How fantastic it was!!
○ What a fantastic party it was!!

There's a grammatical difference between "how" and "what" in exclamations. Here are the patterns you should use:

• How __(adj)__!
• What a __(adj)__ __(n)__!

Some friends of mine opened a flower shop in Aoyama.

http://www.illony.com/taniblog/

When I visited it, I saw these flowers.

× How interesting flowers!
○ What interesting flowers!

To be honest, I usually don't use these patterns, though. I usually say something like:

• Wow! Those flowers are really interesting!

There are lots of other beautiful flowers and arrangements there. You should check it out!