What is the past tense of the word ring?

The past tense form of the word 'ring' is 'rang'. "The teacher rang the bell."

The past tense of "ring" can also be "ringed" since the verb "ring" can be used to mean "to form or provide a circle" or "to surround".

"The police ringed the building."
"The blue-ringed octopus has a poisonous bite."