happen就是动词的原形。
There is no saying what may happen .简直不晓得今后情况将会怎么样。
As it happens ， i have left the book at home .偏巧我把书放在家里了。
Such a thing had never happened before .这样的事，过去从未发生过。
What he tells us is unlikely to happen .他告诉我们的，不像是真事。
I shall be interested to know what happens .我很想知道情况会怎样。
Things are happening so fast around here .这里的事情发展得这样快。
He realized at once what was happening .他一眼就看清了当时的情况。
There is no likelihood of that happening .没有发生那种事情的可能。
I can not remember precisely what happened .我记不准所发生的事情了。
happen就是动词的原形。其第三人称单数形式为happens，过去式和过去分组happened。
例如：We cannot say for sure what will happen.
我们说不准将会发生些什么。
She wondered what would happen if her parents found her.
她不知道如果父母发现了她会怎么样。
If we had been spotted at that point， I don't know what would have happened to us.
如果那时我们被发现了，我不知道我们会有什么后果。
We happened to discover we had a friend in common.
我们碰巧发现我们有个共同的朋友。
