NO.1
I'm down真的是"我不高兴"吗?
I'm down确实有"我不高兴"的意思,但是这里:
I'm down=我乐意,我同意,我加入
(这是美国人最常说的哦~~)
例句:
You wanna hang with us? -- yeah, I'm down.
你愿意和我们一起出去玩吗?--好啊,我乐意!
How about joining us for dinner?—I'm down.
和我们一起吃晚餐怎么样?—好啊,我加入(我乐意).

NO.2
Let me down是"让我下来"?
正解:你真让我失望!
Let me down=让我失望
(let sb. down=让某人失望)
例句:
You won't let me down, will you?
你不会让我失望,对吧?
Don't let your parents down.
别让你的父母失望.

PS: 如果你邀请别人一起玩耍,对方说Sorry, I think I'll let you down(抱歉,恐怕要让你失望了),就是在委婉的拒绝你~~
turn me down顺便了解下?!
turn me down≠把我转下来
=拒绝我
例句:
I asked her to marry me, but she turned me down.
我向她求婚,但是她拒绝了我.

NO.3
Sit down with me难道是"和我坐一起"?
Sit down with me=和我一起讨论
PS: Sit down with sb.确实有"和某人坐在一起"的意思,但是在工作中是说"一起好好讨论"
例句:
Sit down with me and we'll figure out the solution.
和我一起讨论一下,找出解决方案.
