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 listen to the way "happen" is used with the word "what." Remember: When a question word serves as the "subject," don't use a helping verb.
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.)
In the next set of questions, use a helping verb because the question word is not the subject:
When did that happen? (The word "that" is the subject in the question.)
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: