It is put after a verb, noun(+”だ/だった”), i-adjective or na-adjective(+”だ/だった”).
There are two different ways to use it.
First, it can be used to emphasize the thing that is put before ”とは”. It is often used to emphasize the feeling of our shock or surprise. When it is put at the end of a sentence, it is often interchangeable with “なんて”.
And secondly it can be used to define or describe something. It is interchangeable with “というのは”.
- Ken: テスト(は)、どうだった?
How did you do on your test? - Leo: 予想とは違う問題が出たけど、何とか解けた。
There were some questions that I didn’t expect to see on the exam, but I was able to figure out the answers somehow. - Ken:さすが!あんな難しい問題を一時間で解けるとは(orなんて)...(すごい!)
You’re amazing! You could complete such difficult questions in just an hour.
- Leo: 「本音と建前」とは、どういう意味?
What does “本音と建前“ mean? - Ken: 「本音」とは、本当の自分の考えで、「建前」とは、(世間体を踏まえた)表向きの考えという意味だよ。
“本音“ refers to what you really feel, while “建前“ refers to what you are supposed to say. - Leo: そうなんだ。じゃぁ、例えば、彼女が作ってくれたカレーが美味しくなくても、「美味しい」って言う時なんか、(その表現を)使えるの?
I see. So, for example, even if I feel that the curry that my girlfriend made isn’t delicious, can I use that expression when I say “it’s really tasty“? - Ken: うん、そうだよ。
Yeah, that’s right.