I have learnt that some nouns in some situations become plural when we use definte article "the" before them.

Like for example I use verb "has" for our political parties and almost all newspapers do the same thing also. But "sometimes" these very newspapers use "have" for it! To see one such example yourself please go through the compressed link; it is safe, it won't take much of your time as the needed part is highlighted in yellow http://to.ly/6aJE . For the same party I've seen "The Congress have become...' Other example is "The plant must also have sunlight" Plant is not singular here anymore!!

So my question is how do we know a noun has become a plural because of the definte article? Is there some way of knowing it? Hope you have understood my problem. Please help me understand it now.

Thanks and regards Emotion: smile
Hi Razer,
You might be referring to collective nouns which can be used as either singular or plural. The choice of article is secondary to that decision.

The latitude in choice of singlar vs plural is a major difference between British and American English. You can read more about it here .
Razer"The plant must also have sunlight" Plant is not singular here anymore!!
Plant is singular in your example.
There is a modal (auxiliary verb) here - must. Modals do not change form and are always followed by the infinitive form of the main verb.

I must have sunlight.
You must have sunlight.
The plant must have sunlight.

Without the modal, the verb is inflected:

The plant grows best in full sunlight.
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Thanks, it helpsEmotion: smile. I've confused too many things...
Also, in American English at least, there is a somewhat archaic way of speaking that involves using a singular to represent a collective, as in the dog must have water, which could speak to an individual dog, or it could mean that dogs must have water in a general sense. So, occasionally, when you see an example like your plant, the plant may represent all plants, though it is only an individual plant.