how ...!, what ...!
In this construction,
Verb
Place
黒板に
What did you write on the blackboard?
彼は
What did he answer?
This grammar is strictly a casual, spoken equivalent to 何と. It cannot be used with non-communication verbs; for example, you cannot use it to ask 'what will you eat'.
This formation is used to ask for the name or title of a person, place, or thing. It translates to 'what is the name of' or 'what is called'.
Noun
Place
これは
What is the name of this flower?
What is the name of the company you work for?
This formation is used to make an exclamation expressing strong emotion, such as admiration or surprise. It translates to 'How ~!' or 'What a ~!'. It is the spoken equivalent of なんと ~ んだろう.
い-Adjective
Place
How hot it is!
このケーキは
How delicious this cake is!
な-Adjective
Place
How quiet it is!
この町は
How convenient this town is!
Noun
Place
What a genius!
What a good song!
Using んだろう adds emotional emphasis and completes the exclamation. While speakers might occasionally drop it in casual speech and stop at the adjective or noun, including it clarifies the exclamatory nature of the sentence.