If I say:
Your heart just shrank two sizes too small.
Is the verb shrank correct as is? Or should it be in participle form?
Your heart just shrunk two sizes too small.
Which one would be correct in these two cases?
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You would normally use the past tense. There is no auxiliary there.
However, there is historic confusion about whether the past of shrink is supposed to be shrank or shrunk. Shrunk as the past dominated through the 18th century, and remained common during the 19th century; it still survives today as a minority alternative to shrank. Sir Walter Scott used shrunk in Ivanhoe and in Betrothed, both during the 19th century, and you have but to consider the 1989 film, Honey, I Shrunk the Kids, as one modern example of people using shrunk instead of shrank as the past for shrink.
That means that both your examples can be generated by native speakers, although some may think one or the other of those two sounds funny to them. Saying “he shrunk something” instead of “he shrank something” might seem to have a certain rustic sound to it today, but it has a long history. It might be best to avoid in formal writing except for related speech, as many people think only of shrank for the past form of the verb, and so it might be perceived as a careless error.
Writers who used shrunk for the past often used shrunken for the past participle; Scott is one example of such. Today, however, the longer shrunken participle form is now usually reserved for adjectival uses, although it can still occasionally be found causatively, as in “to have shrunken his head” as opposed to “his trousers have shrunk”. (There was once an corresponding causative, to shrench something, that went along with the non-causative to shrink, but which is now obsolete. This shrink/shrench pair followed the same model as drink/drench and stink/stench.)
This sort of variation between which one is the past and which the past participle is particularly common with strong verbs that have a vowel change from i to u in them, such as sing/sang/sung, which once used sung as the past. Similarly, sunk long served as the past of sink, and Johnson in the 18th century called it the normal form, ascribing to sank the label “anciently”. Indeed, the OED allows either sank or sunk for the past, and it allows either sunk or sunken as the past participle. With stink, using stunk for the past persisted through the 19th century before settling down to modern stank and had stunk. The common drink/drank/drunk/drunken has a similarly checkered past.
The OED gives as the etymology of shrink the following: