An adverb used to express surprise or emphasis (as in "what/how ..."); can mean "how" or "what" in exclamations or questions.
何と大きな家だろう。
What a huge house!
何と書けばいいかわからない。
I don't know what I should write.
彼は何と答えたの？
What did he answer?
An exclamatory adverb expressing surprise, admiration, or emphasis ("what...!"/"how...!").
何と素晴らしい演奏だったのだろう。
What a wonderful performance that was!
何と優しい人なんだろう。
How kind a person he is.
何と、彼が約束を守った。
Well, believe it or not, he kept his promise.
Used to indicate surprise or unexpectedness ("surprisingly", "to my amazement", "believe it or not").
何と彼はその会社の社長だった。
To my surprise, he was the president of that company.
何と、あの古い店がまだ営業していた。
Surprisingly, that old shop was still open.
何と彼女が合格していたとは信じられない。
I can't believe she passed, of all things.
An interjection expressing surprise or admiration ("oh my", "wow", "well!"), often used alone.
「彼が優勝したよ」「何と！」
"He won the championship." "Wow!"
何と！それは予想外だ。
Oh my! That's unexpected.
何と、こんなところで会うとは。
Well well, to meet you here of all places!
An interjection used when asking for confirmation or someone's opinion (softening/introducing a question like "well, what do you think?").
この計画で進めてもよろしいでしょうか。何とお考えですか。
May we proceed with this plan? What do you think?
では、この日に決定してよろしいですか。何と？
So, is it all right to decide on this date? What do you say?
その案に賛成していただけますか。何とお願いできますか。
Would you agree to that proposal? Could we have your approval?