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In another question I asked a question about this sentence

For example these compounds are appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

As a side node, the answer corrected it to:

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

But I asked could it be:

For example these compounds are appropriate replacements for volatile organic compounds (VOCs) due to their low vapor pressure.

Because the subject is plural?

It seems both sentence are possible, right? if yes why?

I felt after are I need a plural case, isn't it always right?

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    You could use the singular or the plural there, or avoid it entirely: These compounds can be used in place of VOCs. – Tᴚoɯɐuo Jan 22 '17 at 15:40
  • @TRomano but why I can use both? – Ahmad Jan 22 '17 at 15:42
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Both sentences are possible.

In a sentence with a linking verb (or copular verb)--that is, a to-be verb, there are nouns on both sides of the verb: noun (A) --- is/are --- noun (B)

If the noun that comes before the verb is singular, then the noun that follows the verb must also be singular.

If the noun that comes before the verb is plural, then the noun that comes afer the verb can be either singular or plural, depending on whether the speaker/writer prefers to treat the noun that follows the verb as a singular unit or as members of a group.

A ---> B (A is singular, so B must also be singular)

AAA ---> B (A is plural, so B can be singular or plural)

or

AAA ---> BBB (A is plural, so B can be singular or plural)

A child is a blessing.

Children are a blessing.

Children are blessings.

A vote is an instrument of democracy.

Votes are an instrument of democracy.

Votes are instruments of democracy.


pronoun reference side note

Incidentally, try to avoid using pronouns in ways that might create confusion about which nouns the pronouns are intended to replace.

In the sentence

For example these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to their low vapor pressure.

it might not be clear whether "their" refers to "these compounds" or to "volatile organic compounds."

The usual rule taught in English grammar classes is that the pronoun should refer to the noun that it most closely follows (or precedes). In this case, the noun that "their" most closely follows is "volatile organic compounds," but from the context of the sentence, I think you intended for "their" to actually refer instead to "these compounds." To minimize the possibility of misunderstanding, it would be better to write the sentence as:

For example, these compounds are an appropriate replacement for volatile organic compounds (VOCs) due to these compounds' low vapor pressure.

or

For example, due to their low vapor pressure, these compounds are an appropriate replacement for volatile organic compounds (VOCs).

Confusion over pronoun reference is more of a problem in written English than spoken English. Usually in spoken English, there are enough clues from the context for the speaker's intention to be clear.

In written English, especially in academic or technical writing, the goal is usually to write in a way that makes the meaning as clear as possible, without ambiguity; so more care needs to be paid to pronoun reference than in spoken English.

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