what happen后面+什么?

The verb "happen" is an intransitive action, with a versatile range of applications. It primarily serves to convey the occurrence of an event or a change in a situation. Here are four key ways "happen" is employed:

Reporting Events: To express that something occurred in a specific location or time, you can use the structure "sth. happens + place/time." For instance, "The story took place in 2003" or "An accident occurred on that street."
Describing Events to a Person: When an event occurs to someone, you use "sth. happens to sb." For example, "A car accident happened to her this morning" or "What happened to you?"
Accidental Actions: To indicate that someone unexpectedly performed an action, use "sb. happens to do sth." As in, "I happened to meet a friend on the street yesterday."
Chance Occurrences: "It happens/happened that..." can be used to express a surprising or coincidental event, such as "Brian and Peter were home that day, which was a coincidence" or "He had to attend a meeting, which happened to be that afternoon."

Remember, when "that" is the subject in the "It happens/happened that..." construction, it can be替换为 "sb. happens to do sth." structure, like "Brian and Peter happened to be at home that day."