"They're using a cell phone" vs. "They're using cell phones"

So, this is difficult.

Imagine you are reading a recounting of a historical event.

"The Romans are armed with a short sword known as a gladius which they use to great effect."

Obviously the Roman army has more than one sword but we are using the singular of Gladius here. But there is no way to be certain if that other than context.

I would recommend avoiding phrasing like this and using context to decipher it when you come across it. It is grammatically correct however.

The original post includes 5 inter-related questions and 2 split examples... Making this tricky usage very hard to solve with examples.

So here is a principle: "If the occurrence described is generalized, then the plurality of objects is also generalized."

Here is an example of a specific description: "Right now people in the next room are using a phone."

Since the description is specific to 1 occurrence, the plural "people" and singular "phone" should be treated literally. There is 1 occurrence of multiple people using 1 phone.

Here is an example of a general description: "Once you needed a computer to go online, but now people are using cell phones."

Since the situation is generalized the details involved are also generalized. The speaker references both plural "people" and plural "cell phones" emphasizing the fact that this description is not specific. They don't care about the exact relationship between number of people and number of phones.

For reading and listening to English watch for "specific or general" indicators. Not everyone will use them consistently, but it is common enough to serve as a guideline.

For speaking or writing, make it a goal to use good "specific & literal" or "general and non-literal" indicators:

"I saw 2 men, each carrying a backpack." { specific & literal, each man has a backpack}

"I saw 2 men carrying a backpack." { specific & literal, without a clarifying word like "each" this means they are both holding the same backpack. }

"I have often seen men carrying backpacks" {general & non-literal, there was more than one occasion, there was more than 1 man, there was more than 1 backpack...}

There is useful discussion about when to use and when not to use the distributive singular in the article below:

Do we use the singular or plural? Sometimes, it depends.

By Jennifer Rappaport , MLA Style Center (reformatted and slightly modified; comments added):

The Distributive Plural

When each part of a plural subject possesses something individually, the thing possessed is generally used in the plural as well. For example:

• The two women blew their noses.

Each woman possesses one nose, so, logically, two women possess two noses. Some usage experts call this type of plural “the distributive plural” (Quirk et al. 768).

The Distributive Singular

But in several cases, the thing possessed should be [I'd say often should be; EA] in the singular.

(1) To Indicate Joint Possession

For example, use the singular when two people possess something jointly:

• While traveling together, the two women got lost and consulted their map.

(2) To Convey Universal, Abstract, and Figurative Ideas

Wilson Follett remarks that the thing possessed also “remains in the singular when what is plurally possessed is universal, abstract, or figurative” (211). So if, after blowing their noses, our two women celebrated with a bottle of wine, we might say,

• The two women toasted their health (universal).

If the doorbell rang while they were drinking the wine, we might say,

• The two women were led by their curiosity to open the door (abstract).

But if no one was there when they opened the door, we might say,

• The visitors wanted to get something off their chest but had a change of heart (figurative).

[I'd personally classify the above three examples as (a) abstract-particular – 'Together they toasted life' is abstract-universal; (b) abstract-particular; (c) figurative (two cases). But this section does not take into account the common conversational/rhetorical convention of addressing the single member of an audience, giving a more immediate, less detached feel:

• [to a male US or UK audience] "Be more thoughtful. Kiss your wife when you get home tonight!"
• "Girls, take your ¼" chisel in your weaker hand and your mallet in the other."
• "Be sure to exercise regularly. Swim. Run. Take a long walk. Ride your bike more often."
• 'Reader, I married him.' {this an actual quote}]

(3) To Avoid Ambiguity

Quirk et al. observe that sometimes the singular is needed if the plural would be ambiguous (768):

• We asked the children to name their favorite animal.

If they were asked to name their favorite animals, the children might not be sure if they should name more than one. To make clear that each child, rather than the group as a whole, should give an answer, we could revise as follows:

• We asked each child to answer the question, What is your favorite animal?

Mind Your Nouns

As Words into Type warns, when the sentence has more than one noun, you must be careful to use the singular for the correct noun[/s] (357):

• You should have seen the expression on their faces when they heard the news (not the expression on their face).

Works Cited

• Follett, Wilson. Modern American Usage: A Guide. Revised by Erik Wensberg, Hill and Wang, 1998.

• Quirk, Randolph, et al. A Comprehensive Grammar of the English Language. Longman, 1985.

• Words into Type. 3rd ed., Prentice Hall, 1974.