disappoint 和 let down 的差別在哪裡？如果不好說明，請提供一些例句。

@lerenteleven
Let down is colloquial. Let down and disappoint can be synonyms, but they're not always.
Let down means that you fail to do something people rightly expect of you. You are let down by someone.
Disappoint is similar but refers more to the feelings of the person who is let down. You are disappointed in someone.
But there's a considerable degree of overlap.

He disappointed his teachers by failing his exams. His teachers were sorry that he failed his exams.
He let his teachers down by failing his exams. This has more of an idea that he had promised his teachers he would pass his exams. His teachers felt he had failed them personally. They weren't just sorry that he'd failed his exams.
He said he would give us a lift, but he let us down. He said he'd give us a lift, but he didn't give us one.
We wanted to go on a picnic, but the weather let us down.
If you think I'm going to help you, you'll be disappointed. I haven't said I'm going to help you.
I'm sorry to disappoint you, but you haven't won the prize. I wasn't obliged to award you the prize.
The orchestra gave a disappointing performance. The performance wasn't very good.

Disappointing can be used adjectivally, unlike letting down.

You can be disappointed with or by something.
We're disappointed with our new car. I'm disappointed by your refusal to help your friends.