The plural of "heir" is "heir". There are many other words like this. Is there any rule how we can understand when a word cannot have a different plural form? Or do I have to mug up those words? Please help me.

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    The plural of heir is actually heirs - For nouns like sheep where the plural is identical, I'm not sure of any rule - lots of them are animals (deer, sheep, fish) but that isn't much help. I fear it is something one must simply learn. – David Hall Jul 13 '13 at 16:08
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    I don't think there are that many of them. Here's a list of the ones you're most likely to need (which I notice includes fish, pike and salmon, but not carp). – FumbleFingers Jul 13 '13 at 16:14
  • @FumbleFingers: That's meant to be a list of examples. I don't know why you'd categorize them as "the ones you're most likely to need." It's a pretty good start, but hardly exhaustive. As examples, that list omits squid, plankton, spacecraft, cannon, buffalo, and clippers. To the O.P.: you can probably notice some patterns (aircraft, spacecraft, watercraft, e.g.), but, there's no mnemonic that will cover all of them. Sorry, but you'll have to mug up :^) – J.R. Jul 13 '13 at 20:40
  • @J.R.: I assume OP meant hair, not heir. At that level, you probably don't need to worry about many words. David's right that farmed/food animals (deer, sheep, fish, squid) loom large. There aren't that many other types that are both common and consistently treated as both singular and plural (aircrafts, for example, is making considerable headway). – FumbleFingers Jul 13 '13 at 22:41

There is no rule determining whether or not a noun's plural form is the same as its singular form. As the comments point out, many of these words are words for animals. Another trend I have noticed is that loanwords from Japanese often don't change in the plural (two kanji, three Pokemon, ten anime), though sometimes they do (six ninjas, eighty futons).

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