Plural nouns not ending in s should have add an 's (for example, the alumni's contribution).

Plural nouns ending in s only need an apostrophe (for example, the horses' food).

Singular common nouns ending in s should add an 's unless the next word begins with an s (for example, the witness's answer or the witness' story).

Singluar proper names ending in s only need an apostrophe (for example, Dickens' novels).