Does this rule work for all aphostrophe rules:
If the word ends with a "s" add a apstrophe( ' )
eg.Mens' bathroom
And if the word doen't end with a "s" add a aphstrophe ( ' ) and a "s"
eg.My sister's bed room

thank yas

No, mens' bathroom is wrong. It's men's bathroom.

The placement of apostrophes is explained here.


Here's a 100% rule:

(1) Take your word (regardless of its ending or its number, i.e. singular or plural).
(2) Add es
(3) Replace the e with an apostrophe.
(4) If you're left with an s's ending, delete the last s.

men > menes > men's
cat > cates > cat's
cats > catses > cats's > cats'
people > peoplees > people's
ladies > ladieses > ladies's > ladies'

(This works for everything because the possessive form used to be shown by adding es to everything.)