A combination of factors has conspired to make a bad situation worse.

There has not been any wind or rain to talk of for over a month in Milan - so there has been nothing to disperse the smog.

The situation is not helped by Italians' love affair with the car - particularly the fuel-efficient, but dirty, diesel-engine variety.

For weeks the big industrial cities of northern Italy have been banning cars in ever more draconian ways, to try to cut pollution levels. It has not worked.

The first sentence of the above is flawed. It should be ' A combination of factors have conspired to make a bad situation worse'. Am I wrong?
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The subject of that sentence is "combination," so the verb should be the singular "has." Test it by omitting the prepositional phrase "of factors" from the sentence. Of course, if "factors" were the subject, as in "The factors have conspired . . . ," then the verb would be the plural "have."

A combination of factors has conspired to make a bad situation worse.

I can't take in your point.

A combination of factors is the subject. In other words those 'combinations of factors' led to happen something.

I don't know whether you are able to add some more to show the subject is the word 'combination'.
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Consider: The combination of blue and yellow makes green.
And: Mix the blue and yellow. The combination makes green.
And: Blue and yellow make green.
"Of factors" is a modifier of the subject, not part of a subject phrase.
So many factors came together. Their combination has conspired . . .
I would agree. The literal sense is that the combination of factors makes the situation worse.

Possibly the writer is using 'combination of factors' very loosely, though, to mean simply 'a number of factors'. With 'number of factors', some people would use 'have', others 'has'.

But if we do choose 'has', we then have the problem of a 'combination' (1 party) 'conspiring' (which requires at least 2 parties).

Possibly the writer is also using 'has conspired' very loosely.

Combination is the end product or result of multiple processes or actions. The very word demands that we consider it to be made up of more than one item. Even so it is as singular as the word sum or product is for mathmatics, or the word compound is for the joining of elements in chemistry.

I agree that conspired complicates the sentence, but I do not see how a singular subject would ever justify the plural verb form in a sentence such as this.
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In addition consider: The number is infinite. The number of variations is infinite. The number of conspirators is greater than we thought. The number of members is increasing.

I do not agree that "are" is proper in any of these examples. It may be used and accepted by some, but number and combination are singular nouns and demand a singular verb. The simple subject in each of these sentences is number. These are not complex subject phrases. What is infinite?: number. What is greater than we thought?: number. What is increasing?: number. The modifying phrases act as no more than adjective phrases. They are not a part of the simple subject. We might as well consider the adjective "blue" as part of the simple subject "house" in: "The blue house has a swimming pool in the back yard."

Should we accept: "The sum of two and three are five."; "The product of six and two are twelve."; or "The combination of two hydrogen atoms and one oxygen atom make water."? Your argument supports these sentences. Read the simple sentences of subject and verb: "sum are" and "combination make." Shall we let the prepositional phrases influence our judgement? That is what you are doing.

We make a quagmire of understanding grammar by wandering off the path to allow everything remotely plausible. Yes, grammar is dynamic, elastic, pliable. It is regional, national, colloquial. It is common, specific, professional. At the same time it also needs to be somewhat static and resistant to change so that, at least for a few brief moments in time, we can know what is right and what is not. If it is not so, then we may as well forget what is proper and that which is improper. Let us be anarchists and admit any use of our language. The tower of Babel awaits.
I thank both of you for the excellent replies. Now I understand perfectly.
Hello R2005

My comment wasn't meant to imply that I would change 'has' to 'have' simply because a conspiracy requires more than one person (except in certain kinds of mental ill health, of course). I would guess that 'conspire' has been used quite loosely in this sentence, with only a faint echo of its original meaning.

In BrE (which is all I can speak for), nouns such as 'number' can sometimes be used with a plural verb, where the sense requires it:

1. A number of jury members disagree about that particular piece of evidence.

Here it would sound strange and incongruous to say:

2. A number of jury members disagrees about that particular piece of evidence.

(I take it that 'a number of' is used a synonym for 'some' in such cases.)

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