vi.

1. 发生
   Earthquakes occur frequently in this area.

   这一地区经常发生地震。

2. 出现；存在；被发现[Q]
   Do giraffes occur in Africa only?

   长颈鹿是不是只在非洲才有？

3. 被想起，被想到，浮现[（+to）]
   It occurred to me to visit my teacher.

   我想到要去看看我的老师。

   It never occurred to me that he might be in trouble.

   我怎么也没有想到他会遇上麻烦。

   A brilliant idea occurred to me.

   我想到一个极好的主意。

   It didn't occur to him that she would refuse his invitation.

   他没有想到她会拒绝他的邀请。

v.

1. happen; take place.
2. exist or be found to be present in a place or under a particular set of conditions:
   radon occurs naturally in granite.
3. (occur to) come into the mind of.