American Academic English is extremely confusing. One reason is that we have identical written structures to mean different, unrelated things. Another reason is that some of these structures depend on punctuation which you cannot hear. Yet another reason is that some of these forms seem unnecessary if you are coming from another language.
English sticks an S on the ends of words for several different reasons, and some of these (but not all of them) require an apostrophe.
Formal American English always requires a spelling change for for plural nouns. Plain plural never takes an apostrophe.
This spelling change is required even if the result is difficult to pronounce:
Some irregular nouns form a plural by changing something inside the word. These do not take a "s" on the end.
Some languages show possession by position in the sentence or by context. Formal American English always requires the spelling change.
These forms do take an apostrophe. This causes three problems. First, apostrophes do not make any sound, so you cannot write these "by ear." Second, the apostrophe moves to show whether the noun is singular possessive or plural possessive (but the two forms sound identical). Third, irregular nouns which form their plurals by changing letters inside the word form possession the same way singular nouns do.
Many contractions take the form of word+apostrophe+s.
You can stay out of trouble by using full spelling in all formal writing.
Some words (particularly personal names) just have an s on the end. They do not take an apostrophe.
Possession causes trouble with names that end in "s." The grammar world is still undecided about these (though APA style always requires apostrophe+s). With some exceptions (usually biblical persons), we usually go with name+apostrophe+s just as we would with any other noun.
If that last one sounds weird, you have two answers. One is that written English is not the same as spoken English. The other is that you can write around it: "The article by Roberts."
An alternative you often see leaves off the final s (but it still sounds weird because we really say "Robertses"). Pick one style and stick with it. If you are writing in APA style, use the one above, but you will often see this as well:
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Revised 12/20/18 • Page author: Curtis Allen • e-mail: firstname.lastname@example.org.