shrank / shrunk
The past and the past participle of shrink.
The past of shrink is shrank. The past participle is shrunk, after forms of have and forms of be: The shirt shrank. The shirt has shrunk. The shirt was shrunk. Do not use The shirt shrunk for the simple past.
Examples
Use The hose shrank in the heat.
Use The hose has shrunk.
Avoid The hose shrunk in the heat.