If u happen___________ 造句。。意思是

If u happen to be his neighbour ,what will you do.
如果你又恰巧是他的邻居，你会怎么做呢。

happen to do
恰巧……

if you happen to see him, please give him the book.
如果你碰巧能见着他，请把这本书给他。

It you happen to see the person, you will be suprised how beautiful she looks.
假如你见了她本人，你会惊讶她有多漂亮

to see my dog, please call my home.
如果你看见了我的狗，请打电话到我家。