我请客 it's my treat
(1) it's my treat/my treat “我请客”
(2) treat作动词，“请客，款待，招待”
(3) treat sb. to sth. “款待某人某物，请某人吃某物”
例
He's treating her to a meal, but his own portion is small.
他请她吃饭，自己那份点的却很少。
Can we get another bottle of champagne for the table? Don't worry. It's my treat.
能给我们桌再来一瓶香槟吗？别担心，我请客。《生活大爆炸》
我请客 it's on me
(1) on有“由……付钱”的意思
(2) on sb.“由某人支付”，口语
(3) it's on me “我请客”
(4) this is on me “这顿我请”
例
I'll get my purse, and dinner is on me, okay?
我去拿我的包，晚饭我请，好吗？
By the way, this is on me.
顺便说一下，这顿我请。
店家赠送 on the house
on the house “店家赠送，免费”，常用于饭店、酒吧等地方
例
How about a piece of cheesecake on the house?
来块免费芝士蛋糕怎么样？
It's on the house.
这是本店赠送的。
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