Shrank is the simple past and stands alone: "the jumper shrank in the wash." Shrunk is the past participle and needs a helper: "the jumper has shrunk." The film title Honey, I Shrunk the Kids uses the participle where the simple past would be standard — which is a large part of why people are unsure.
The core difference
The same helper-verb pattern as began/begun and drank/drunk.
- shrank — no helper: It shrank.
- shrunk — with have/has/had: It has shrunk.
- shrunken — the adjective: a shrunken head.
Why this one feels uncertain
Because shrunk has been gaining ground as a simple past for a long time, and many dictionaries now list it as an accepted variant. So "it shrunk in the wash" is not the clear error that "I have shrank" is. In careful writing, keep shrank for the simple past.
The third form
Shrunken survives only as an adjective. You cannot say "it has shrunken" in ordinary modern English, but "a shrunken figure" is perfectly standard.
Frequently asked questions
Is "it shrunk" wrong?
It is widely used and accepted as a variant, though shrank remains the standard simple past in careful writing.
Is "I have shrank" correct?
No. After have, has or had you need the participle: have shrunk.
When do I use shrunken?
Only as an adjective — a shrunken head, a shrunken figure. It is not used with have.