Deciding whether to use just the apostrophe or an apostrophe s for a possessive noun, is based on how you would say it, not whether it is singular or plural or already ends with an s. For example:
MDs (It does not take an apostrophy.)
The plural form for the noun box is boxes.The plural possessive form is boxes'.Example: "We need more ink for the boxes' labels."
The plural possessive for mailbox is mailboxes'. Example sentence:They gave us a list of the mailboxes' locations in need of repair.
The plural form for the noun seat is seats.The plural possessive form is seats'.Example: The seats' positions need to be arranged.
If you want a plural, no. Correct:bear, bearscat, catscheetah, cheetahdog, dogselephant, elephantsetc.If you need a possessive, yes it needs an apostrophe. Correct:The bear's claws are large. (singular possessive) The bears' habitat is the forest. (plural possessive)The cat's whiskers are white. (singular possessive) The cats' cages need cleaned. (plural possessive)We can see the cheetah's leg is hurt. (singular possessive) The cheetahs' coats are beautiful. (plural possessive)etc.
Singular possessive: secretary's Plural: secretaries Plural possessive: secretaries'
The plural is relatives. The plural possessive is relatives'.
The plural is lads. The plural possessive is lads'.
The plural form is echoes. The plural possessive is echoes'.
The plural form is suffixes. The plural possessive is suffixes'.
The plural form is Mexicans. The plural possessive is Mexicans'.
The plural form is stepsisters. The plural possessive is stepsisters'.