Why does the plural form of "life" is "lives", while the plural form of "still life" is "still lifes"?

From Wikipedia:

A still life (plural still lifes) is a work of art depicting mostly inanimate subject matter, typically commonplace objects which may be either natural (food, flowers, plants, rocks, or shells) or man-made (drinking glasses, books, vases, jewelry, coins, pipes, and so on).

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    Thank you for asking this question. You saved me from making a mistake (not that I speak of paintings that often, but still... ) – Paola Jun 22 '12 at 19:52
  • Hello Paola, I am pleased to hear you. (I hope 'pleased' is the right word! I wont say 'piacere') – user19148 Jun 22 '12 at 20:00
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    @Carlo_R. A slightly better phrasing might be "pleased to hear from you" since you're not actually physically hearing Paola – simchona Jun 22 '12 at 21:56

I would say still life has undergone reification, which transforms it into a "standalone word". How the subcomponent elements work grammatically doesn't automatically affect how the composite form works.

Effectively it's a kind of neologism - not really "new" today, but a lot later than the original word life with its irregular plural. Neologisms almost always have regular plural and verb forms.

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    The technical term is Reification. The lexical item has been reanalyzed as a separate word, in a separate context, and new words are regular, so it doesn't inherit the irregularities of the parent word. Similar reification accounts for the weirdness of *New English Boiled Dinner and *Toronto Maple Leaves. – John Lawler Jun 22 '12 at 18:18
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    @John: Thanks for that! - I knew what concept I was getting at, but I half-thought I had the wrong word. Will discard "grammaticalisation". – FumbleFingers Jun 22 '12 at 18:21
  • I was honestly unaware it was actually two words (I've always heard it pronounced, never read it). – T.E.D. Jun 22 '12 at 18:33
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    Grammaticalization refers to a different phenomenon. – John Lawler Jun 22 '12 at 18:37
  • That Wikipedia article is about a completely different sense of the term "reification"; I think you might as well not link to it. – ruakh Jun 22 '12 at 19:41

Irregular plurals and irregular deverbal nouns (i.e., nouns formed from verbs, like house /hauz/ from house /haus/) are frequently restricted in the way that your example and Barrie's (computer) mouses illustrate: the irregular form is only used for the most common meaning, with a fully regular form being used in neologisms and other variants.

Other examples are:

  1. I might wonder when my computer was hard-drived, but not hard-driven.

  2. If I hit someone with the sheath of a sword, I might be said to shea[θ] them, but not to shea[ð] them (which only means "to put a sword in its sheath").

  3. When the evil witch gets squashed with a house in The Wizard of Oz, she might be said to get hou[s]ed, but not hou[z]ed (which only means "to provide with housing").

  4. When bad measures street cred, its superlative is baddest, not worst.

  5. The past tense of input is, for many people, inputted (as in Who inputted these data?), even though the past tense of put is just put.

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  • Nice answer, but the evil witch is hou[s]ed at the beginning of the wizard of oz. – Jakob Weisblat Jun 27 '12 at 17:48
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    @Jake223 Thanks. Simpler just to say "in". – Daniel Harbour Jul 3 '12 at 20:12

Because still lifes are not the same thing as still lives. Similarly, mouses are not the same thing as mice.

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  • What are 'mouses' then? – Mitch Jun 22 '12 at 18:38
  • I've encountered mouses as a verb form, but not as a plural noun. – Marthaª Jun 22 '12 at 18:40
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    @Mitch: Computer pointing devices. – Barrie England Jun 22 '12 at 18:48
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    Oh. OK. I always called them 'mice', but I guess I've heard the alternative you gave, too. – Mitch Jun 22 '12 at 19:32