Please add the correct punctuation mark (apostrophe) to the following question:   My stepbrothers car needs new upholstery and new tires. 

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mizzwillie eNotes educator| Certified Educator

In punctuation marks, the apostrophe seems to cause the most trouble for students.  The concept in question here is the difference among possession, plurals and the s in third person singular verbs.  The correct placement of the apostrophe is between the r and the s  as in stepbrother's car.  The apostrophe here shows that the car belongs to the stepbrother.  The s in needs is there to show that the verb agrees with the third person singular subject; a simpler example of this is "he runs."  The s at the end of tires is to show a plural noun meaning that the car would need more than one tire.  Plurals do not need apostrophes; therefore, the only apostrophe required in this sentence is the one indicating possession--that the car belongs to the stepbrother.  The link below will help you learn how to use apostrophes, and since they are often used incorrectly, this tool could help you a great deal.