Oh no, I forgot your birthday! Hey, just/only kidding!
啊,我把你的生日忘了!嘿,开玩笑的!
You won first prize? You're kidding! (= I'm really surprised.)
你得了第一?开玩笑吧!
I'm just kidding you!
我不过和你开个玩笑!
He says there's a good chance she'll come back to him but I think he's kidding himself.
他说她很可能会回到他身边,但我觉得他是在自欺欺人。