How do you know when to use "hanged" or "hung" in a sentence? This applies to any other words similar to this.
In modern American English, "hanged" is only used when referring to death by hanging, whereas "hung" is used as the general past tense of "hang".
While some people still use both words interchangeably, a majority of modern grammar and style professionals insist that "hung" is the appropriate general past tense of "hang", whereas "hanged" is only used when referring to death by hanging (in either a murder or suicide sense).
A good way to remember this is using a short mnemonic device:
"Curtains are hung, but people are hanged."
I can't think of another example of a verb similar to this; most past-tense differences occur between British and American English (learnt vs. learned, for instance).