1. Beth's and Mary's car is in the shop.
In this sentence there’s one car. To show joint ownership an apostrophe plus “s” is only needed at the last part of the compound noun. The apostrophe plus “s” needs to be omitted after the word Beth.
Beth and Mary’s car is in the shop.
2. Tom smoking bothered the residents.
The word smoking is a gerund. When a noun is used immediately before a gerund the noun must be possessive. An apostrophe plus “s” is needed after the word Tom.
Tom’s smoking bothered the residents.
3. The dog and cat's ears were infected.
An apostrophe plus “s” is needed after the word dog to show separate ownership. Both the dog and cat own their own pair of separate ears.
The dog’s and cat’s ears were infected.
4. Explain the placement
In this sentence there’s one car. To show joint ownership an apostrophe plus “s” is only needed at the last part of the compound noun. The apostrophe plus “s” needs to be omitted after the word Beth.
Beth and Mary’s car is in the shop.
2. Tom smoking bothered the residents.
The word smoking is a gerund. When a noun is used immediately before a gerund the noun must be possessive. An apostrophe plus “s” is needed after the word Tom.
Tom’s smoking bothered the residents.
3. The dog and cat's ears were infected.
An apostrophe plus “s” is needed after the word dog to show separate ownership. Both the dog and cat own their own pair of separate ears.
The dog’s and cat’s ears were infected.
4. Explain the placement