The corrected sentence is: "Mom's and Dad's Volvo was demolished, but both of them were only slightly injured in the seven-car crash."
Changes Made:
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Capitalization: "mom's" and "dad's" should be capitalized as "Mom's" and "Dad's." Proper nouns referring to specific people (like "Mom" and "Dad") are typically capitalized when used as names.
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Singular vs. Plural: Changed "was" to "were" because "both of them" is plural, and thus requires the plural form of the verb.
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Hyphenation: Added a hyphen in "seven-car crash" to indicate that "seven" is an adjective describing the noun "car," and when two or more words modify a noun together, they are often hyphenated.
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Punctuation: Added a comma before "but" to properly separate the two independent clauses.
These changes improve the grammatical accuracy and clarity of the sentence.