meaning and usage of the following question words

what happened

• its used when you want to know what occured
• used in past tense

When?

• at what time.
• at or on which (referring to a time or circumstance).
• present tense

where?

• in or at what place?:
• in what position or circumstances?
• present tense

who?

• what or which person or people.
• said to introduce a clause giving further information about a person or people previously mentioned.

Why?

• for what cause, reason, or purpose
• the cause, reason, or purpose for which