The phrase "what happen" is generally incorrect in standard English because the verb "happen" needs to be conjugated properly depending on the subject and tense. The correct forms are:

• "What happened?" - This is used to ask about something that occurred in the past. It is the proper past tense form and is grammatically correct for inquiring about past events

• "What happens?" - This is used to ask about something that occurs regularly, habitually, or in the present tense

The phrase "what happen" without an -s or -ed ending is not standard English because the verb "happen" in the third person singular present tense requires an -s ending ("happens"), and the past tense requires the -ed ending ("happened")

. In summary:

• Use "What happened?" to ask about a specific event in the past.
• Use "What happens?" to ask about present or habitual events.
• Avoid using "What happen?" as it is grammatically incorrect in standard English