The word "happen" is used for activities, events, accidents, and surprises. It's kind of a tricky verb to use, so you really have to study it. First, let's look at how to use "happen" with the word "what":
What happened? (Ask this question when you first learn that there's a problem.)
What happens here every day? (Use this kind of a question for general activity.)
What happens if I put these two things together? (Ask this kind of a question when considering possibility.)
Notice that when you use the question word "what" with "happen," you don't use a helping verb. In the next set of questions, you do use a helping verb:
When did that happen?
Where does this usually happen?
How can this happen?
Why hasn't that happened yet?
The word "happen" is often used in the present tense, the past tense, and the future tense:
That happened yesterday. (past tense)
This happens every day. (present tense)
It won't happen until tomorrow. (future tense, negative)
"Happen" is also found in many expressions:
Hey, what's happening? (Hi. How are you?)
This can't be happening. (This is really bad.)
We have to make this happen. (We have to work hard to be successful.)
This party is not happening. Let's go. (This is not fun or exciting.)
That is not going to happen. (I will do anything to prevent that.)
Watch this video for more examples of how to use the word "happen" for past activities: