I am a new English learner and need some assistance with understanding when to properly use an apostrophe for showing possession.
I have an example sentence:
Sophie, Brock, and Hannah all own separate headphones for their iPhones.
In this sentence I am unsure whether to place an apostrophe at the end of headphones, iPhones, or if I even need one at all.
I would just like a little clarification on this topic.
Thanks!
In that sentence, you don't need any apostrophes.
As you probably know, the construction apostrophe+s is a way to build the genitive case of nouns in English. This is a remnant of an old case system that has been largely lost in modern English.
According to CGEL, there are six types of genitive construction (p. 467):
[41] i [Kim's father] has arrived. [I: subject-determiner]
ii No one objected to [Kim's joining the party]. [II: subject of gerund-participial]
iii Max's attempt wasn't as good as [Kim's]. [III: fused subject-determiner-head]
iv She's [ a friend of Kim's]. [IV: oblique genitive]
v All this is Kim's. [V: predicative genitive]
vi He lives in [an old people's home]. [VI: attributive genitive]
But in your sentence, the noun headphones appears as the head of the noun phrase (NP) separate headphones for their iPhones (here separate is an adjective, a pre-head modifier of headphones); and the NP separate headphones for their iPhones, in turn, appears as the direct object of the verb own. We see that the function of the direct object is not one of the six genitive constructions listed above.
Instead, the direct object function is normally associated with the accusative case. In English, the accusative case survives only for some of the personal pronouns and the relative pronoun who: (I → me, he → him, she → her, we → us, they → them, who → whom). This is why we say I saw her (but not *I saw she) and She saw me (but not *She saw I). In particular, you would say She owns me (me = the accusative of I), not *She owns I (I = the nominative of I) or *she owns my (my = the genitive of I).
But nouns like headphones do not inflect for the accusative case, and so, when functioning as direct objects (or as heads of NPs functioning as direct objects), they are kept in the plain case.1
1The accusative case is normally contrasted with the nominative case, which is the case the noun has when it appears as the subject of a finite clause (i.e. not as the subject of a gerund-participial clause). But since for English nouns there is no distinction between the nominative and the accusative, CGEL wants to 'neutralize the nominative-accusative opposition', and therefore refers to the nominative/accusative form of the noun as the 'plain case' of the noun. In contrast, for personal pronouns, CGEL retains the notion of the nominative and the accusative.
Incidentally, in many cases, you can use the behavior of pronouns to figure out if you need to put your noun in the genitive. Just replace the relevant NP by a personal pronoun. If it turns out that the pronoun must be in the genitive (my/mine, your/yours, his, her/hers, its, our/ours, their/theirs, whose), then you need to place your head noun in the genitive. Otherwise, your head noun should remain in the plain case.
Having said that, you do have one genitive in that sentence: their. This is the genitive of the noun they, and it is not formed by adding 's but by this more substantial change.
Here are some constructions where you would use the genitive of the nouns:
Sophie's headphones
Sophie's, Brock's, and Hannah's headphones
the three iPhones' headphones (= the headphones, probably multiple, that belong
to the three iPhones; grammatical, but probably
few people would actually put it like that)
Sophie's iPhones's headphones (again, grammatical, but probably few people would actually put it like that)
In your example no apostrophe is required.
You would required one if you wrote the iPhones' headphones, indicating more than one iPhone; or, the iPhone's headphone/s indicating a single iPhone.
Similarly, you would need one after any of the names: Sophie's headphone/s.
But the best solution is to google The use of apostrophes. You will find numerous sites - such as the one below - that offer both explanations and examples that ought to resolve your problems.
https://grammar.yourdictionary.com/punctuation/apostrophe-rules.html
This sentence does not show possession through apostrophes, but by the verb, "own". An example using an apostrophe is, "It's Hannah's phone." BTW, the apostrophe in "it's" is used in the contraction of "it is", not the possessive pronoun, "its", which doesn't have an apostrophe to distinguish from the contraction.
See an amusing article on the Apostrophe Protection Society.
The answers posted so far are great, but just to offer a succinct take, neither the headphones nor the iPhones are the owners of anything in this sentence; the girls are, and their ownership of the iPhones is implicit in the possessive pronoun "their".
