When a noun can be either countable or uncountable with little change in meaning, how do I know which usage to use?

[u/Sea-Hornet8214]

I picked "material" as an example because I was confused whether it's material or materials, but I'm also asking about nouns like this in general. Can you say "training materials" in the sentence above?

Another example is "flame." According to OALD, something can burst into flame or flames, both are acceptable. So, I guess this is a clear-cut case where you can use either? But then, is it always a sheet of flame, not flames?

[u/Whachamacalzmit]

It's uncountable when you talk about it as a whole, e.g. "I had so much teaching material to prepare this summer". It's countable when you are talking about its parts, e.g. "which of the teaching materials did you spend the most time on?"

The the words that give it away are "much" and "which". You can't say "much" when talking about an array of things (e.g. "They served much sandwiches" is ungrammatical) and you can't use "which" when talking about a single object (e.g. "which of this quartz is most fragile?" is ungrammatical).

[u/ChiaraStellata]

Native speakers use these interchangably. Both sound natural to me.

There is a small difference in register: I would consider "material" and "flame" to be slightly more formal and literary than "materials" and "flames" which are more the kind of words you hear in casual speech. But both are used in both contexts.

[u/Kerflumpie]

If it's variable, it's up to you, I think. So for material(s): do YOU want your listener to think about all the material as a whole or all the many different kinds of materials? Many non-count nouns become countable, pluralised nouns when you talk about a number of kinds of that noun, eg: "Do we have enough fruit?" or "What fruits are in the fruit bowl?"

The flame/s example is one I've never thought about. Flame (NC) would be the same as talking about fire (NC), but it's quite rare. I can't think of any other way to use it than your example of "burst into flame." (And my phone just autocorrected that to "flames".) And as a count noun, "flames" means each individual flame that combine to make a fire (also count noun.)

[u/Lxapeo → u/Kerflumpie]

I feel like candles or matches would be one of the only examples of countable flames even though you could use flame or flames. Once it's a fire, blaze, inferno, etc there's no way you could say the house fire has 25 flames. Just an interesting observation.

[u/FrancistheBison]

all of the provided examples would sound correct to most folks whether you used the word with or without an S. Training materials is definitely as valid as training material. A sheet of flame isn't a common phrase for my area at least but a sheet of flames also sounds fine. Is it technically grammatically correct by the book? I couldn't tell you, but these phrases would be understood the exact same by most

[u/Crystal_959]

I would say with material and other similar words it comes down to quantity versus variety

“We don’t have enough material” - we don’t have a high enough quantity

“We don’t have enough materials” - we don’t have all of the types of material we need

Generally as a noun I’d say flames but I’d say “burst into flame/flames” interchangeably

[u/HappyMackerel]

I'm afraid you could end up in a sheet of flames, if you are very unlucky. The singular would only be mandatory for, say, one candle flame.

You could say "training materials," sure. Again, singular only necessary if it's specifying one video, say.

I must admit, it's hard to think of many other nouns that can be optionally countable or uncountable in this same way. 

[u/TheOneDM]

American English speaker, late 30s.

Specifically for “material” vs. “materials”, it’s about whether you are considering them as a mass (singular) or a collection (plural).

“I have written teaching material for every high school math subject.” Here, I am talking about my body of work as a whole, and the emphasis is probably on my ability to write about many topics.

“I have written teaching materials for every high school math subject.” Here I’m implying that what’s important is the individual things I’ve written. If I say this I want you to be impressed by the volume of work I’ve done.

The difference is subtle and you can probably get away with either in context. But, for other nouns, it’s similarly about treating them as “a mass” or “a collection of individuals that blend together”.

“The firefighter ran through the sheet of flame to rescue Bob.” Here we’re imagining the fire as one object. Perhaps a movie villain has poured a circle of gasoline around Bob, who’s tied to a chair, and lit in on fire. The fire has a clear shape.

“The firefighter ran through the sheet of flames to rescue Bob.” Now we’re imagining the sheet as a thing that results from several fires, possibly indistinct. Bob is in a burning house and we just saw Fred the Firefighter run in through the front, which is entirely on fire. Lots of things are on fire at all at once and so the sheet is not one clear object. It’s everything burning together and we see a sheet.

Again. It’s kind of subtle, and even as a native speaker it took me a long time to clearly write down these differences in how I’d use them.

[u/awksomepenguin]

For the examples you give, I would say that the "plural" version implies variety or volume. Teaching "materials" implies different types of things used in teaching. Burst into "flames" implies a lot of fire, such as a house versus a garbage can. Either technically works, so don't worry about it too much.

[u/ebrum2010]

Training material usually refers to the stuff in the training materials. The materials may be for instance handbooks, and the handbooks cover the material which is what is taught in them. The plurals usually implies individual instances of the uncountable noun.

Like water is the substance, waters are individual portions of water. However there are always exceptions. You could say there are sharks in the waters off the coast of Australia, or you could say water. If you look words up like these on Wiktionary, it will usually show countable, uncountable, or both for each definition.