Sentences are:
"The rainforests are in danger." "Rainforests are in danger."
So in this case, I think that they are both correct sentences...but:
"Guitars are musical instruments." "The guitars are musical instruments."
In this case, we are still making generic statements about a plural noun, but the second one sounds bizarre. This for a student. I'm not sure WHY it's awkward. Why do we opt for "The guitar is a musical instrument."
Thanks in advance :)
There are three kinds of generic noun phrases. As I put it in this answer to Ask-A-Linguist in 1997,
- Definite Generic: the +
Singular Noun
The tiger is in danger of becoming extinct.- Plural Generic:
0+Plural Noun[0= Zero, the number; i.e, no article]
Tigers are in danger of becoming extinct.- Indefinite Generic: a +
Singular Noun
*A tiger is in danger of becoming extinct.
(Note that the third one is ungrammatical with becoming extinct. There is a reason for this.)
As for their usage:
The Definite Generic refers to the Prototype of a species, roughly the image we associate with tiger. The tiger, as a prototype, has all the properties of anything we would call a tiger, except that it doesn't exist in an individual physical sense, like all real tigers do. This is a very abstract concept, and its use signals that the speaker is theorizing.
The tiger is big means the speaker believes that "bigness", in some comparative context, is a characteristic property of tigers, that we should expect this to be true of any tiger.The Plural Generic refers to the Norm of a species over its individuals, as perceived, of course, by the speaker, who is unlikely to have conducted tiger surveys, so the "statistics" here are very vague and impressional.
Tigers are big means the speaker believes that, on the average, any tiger is likely to be "big". This doesn't mean all tigers are big, though that's close. This is potentially a less abstract concept, since its use implies a generalization based on experience of several individuals.The Indefinite Generic refers to the Definition of a species -- that is, those properties that are absolutely necessary for anything to be a member. It doesn't work as the subject of any predicate that isn't definitional. But with a definitional property, it's certainly true for any member.
And that's the reason I mentioned above, why the third example is ungrammatical:
being in danger of becoming extinct is not a definitional property for tigers.
A tiger would still be a tiger if there were no danger of extinction.
As for rainforests and guitars:
Added directly from comment:
In the non-generic sense, A rainforest is in danger is not ungrammatical. And The rainforest is in danger is ambiguous, too. Both can refer to an individual rainforest. The generic constructions are simply additional idiomatic uses for articles
A.
The rainforests are in danger.
The meaning of this depends on context, e.g.
"What can you tell me about Brazil?"
"The rainforests are in danger."
This means that all the rainforests in Brazil are endangered.
"What can you tell me about our planet?"
"The rainforests are in danger."
This means that all the rainforests on Earth are endangered.
B.
Rainforests are in danger.
This means that some rainforests are in danger but we don't know which or how many.
C.
The guitars are musical instruments.
This makes a statement about a particular set of guitars. Presumably we think that they might be fake or toys and someone is telling us that these guitars are actually real instruments.
D.
Guitars are musical instruments.
This tells us what kind of thing guitars are. It is a partial definition.
Discussion
There is a difference in type between 'to be a musical instrument' and 'to be in danger'. The first relates to a noun and the second is adjectival. Thus the two sets of sentence aren't directly comparable.
The meaning of most statements in English is dependent on context. Any time you have a statement that "X is Y", and "X" does not identify a specific entity, then identity of X must be inferred from context.
Consider first a statement like "Rainforests are in danger." This simply says that some subset of all rainforests are in danger. From tone and meager bits of context one might infer that the meaning is "all rainforests are in danger", but that meaning is not explicitly stated.
When one says "The rainforests are in danger," the use of "the" tells the listener that context must be consulted to determine which specific rainforests are being indicated. If the surrounding discussion is about, say, South America then it can be inferred that "the rainforests of South America" are being indicated. However, if there is no such context then the "wider context" ("global", in fact) is the general knowledge that there are rainforests all over the world and hence the term is interpreted as "the rainforests of the world".
With guitars, if someone says "the guitars sound lousy" there is no standard "wider context" such as there is for "rainforest", so, outside of, say, a discussion about the local rock band, there is no reference point for "the guitars", and the statement sounds, well, silly. If one says "the guitars are musical instruments" it doesn't sound quite as silly, but it's still clearly not right, for the same reason.
In general, when "the" is used with a plural noun to indicate a generic class, there must be some available context so that the reader/listener can resolve which entities are being identified. Sometimes, for some classes, this "context" can be "global" in nature and not be the immediate context of the sentence, or even the wider context of the article or the technical domain of the article. In other cases there is no such "global context", and a generic "the" will end up being ambiguous.
I mean - You could say "The rainforests are forests". This seems to make sense grammatically, but is equally meaningless.
– Stu Cooke Oct 11 '15 at 22:45