And for plural "s": if the word normally ends in a "s" or "sh" or "z" sound (or possibly even "zh", i.e. the "j" in French "je") add an "es" instead of "s". Some words are irregular but irregular words by definition don't follow the rules.

The real confusion comes with possessive pronouns. It is "the boy's pet" but it is "his pet" (not #"he's pet"). They're not nouns (they're pro-nouns, i.e. something that is used instead of a noun) so the apostrophe-s rule doesn't apply to them, they just take on special forms.

The confusion really just stems from English regularly allowing contractions like "-'s"/"-'" for "is" and "-n't"/"-'t" for "not" or "-'d" for "would". Pronouns can have contractions as suffixes (e.g. "he's" for "he is" or "he'd" for "he would") but nouns normally can't (#"the boy'd" isn't normally permissible, at least not in writing).

It gets even worse if you consider that whether words can only be contracted additionally depends on their pronunciation (and role) within the sentence. You can't answer the question "Is he dead?" with #"He's.", for example.

Oh, and I haven't even touched upon similar-sounding but entirely different things like "their" vs "they're" -- and I even caught myself accidentally mixing them up while writing this comment.