这是我为什么在这儿的原因。
That's what I'm here for.
What are you studying at Harvard? (你在哈佛大学学什么?)
Economics. That's what I'm here for. (学经济，这就是我为什么在这儿学习的原因。)

2+3等于5。
Two and three is five.
Two plus three is five.
Two and three makes five.
Two and three equals five.

我们有天壤之别。
We're as different as night and day.
We're completely different. (我们俩完全不同。)
We have nothing in