UFO (Unidentified Flying Object)
不明飞行物（飞碟）

例句：
I thought I saw a UFO, but it was really an airplane.
我以为我看到了飞碟，但实际上那只是一架飞机。
 
She told the police that a UFO abducted¹ her in the middle of the night.
她告诉警察，昨天半夜有一艘飞碟绑架了她。

bury one's head in the sand 　　　　　　
逃避现实；拒绝承认现实

例句：
He had suspected for some months that his son was taking drugs, but he had buried his head in the sand and refused to admit it, even to himself.
几个月来，他一直怀疑自己的儿子在吸毒，但他却采取鸵鸟政策，拒绝承认现实。

本句来源：
鸵鸟遇到危险时，总是把头埋在沙里，自己看不见敌人就认为敌人也看不见自己，由此产生这一成语。

I'm afraid I have to leave now. 
恐怕我点走了。

I'm afraid...是一种委婉用语，表示“恐怕……”的意思。
　　
例如：
I'm afraid you are wrong about it this time.
恐怕这次你错了。