I came across a sentence: Some people are wary of using their credit card.
Why not credit cards?
Actually I'd like to ask when do we use a singular noun after THEIR. I've looked through the Internet to find the answer but with no result. There are only skimpy sentences.
Let's look at:
1. Some people are wary of using their credit cards/credit card.
2. A flight attendant should be aware of heir responsibility/responsibilities
3. A student should not forget 'his or her/their' books.(a few books)
4. A student should not forget 'his or her/their' math book.(one particular book)
I don't understand it.Could anybody help and give the rule on this?
"Their" refers to the owner, not the object.
One person can have one thing. The student forgot her book.
One person can have multiple things. The student forgot her books.
Two people can jointly have one thing. Ted and Alice enjoyed thieir vacation.
Two people can jointly have multiple things. Ted and Alice like their neighbors.
There is no need for his/her/their to match the noun that follows.
Also note that "their" is very commonly used for a single person when the gender is not known.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.