Csdrhrt

Are these sentences both correct?

What is the plural of boy?

What is the plural to boy?

Are they interchangeable?

It’s always the plural of something.

What is the plural of “boy”?

To make the plural of "dog" you add the plural ending -s.

“Are” is the second person plural of the verb “to be”.

The word “teeth” is an irregular plural of the noun “tooth”.

What's the plural of “tooth”?

"there is no singular to audience."

This is semantically incorrect - audience is a singular word, the plural of which is audiences - but syntactically, it's fine. If we instead pick a word like sheep, where the same word stands for one or many, then there is no plural to sheep is equally as correct as there is no plural of sheep.

The use of the different prepositions (of, to) gives different inflections to the adjective (singular). We can see this if we add explicit nouns:

There is no plural form of 'sheep'

There is no plural equivalent to 'sheep'

This holds while we are saying there is no.... In all the examples so far provided, we are making the adjectives plural or singular stand in for nouns. If we are asking about the existence of a plural, then form makes sense as the noun, but so might equivalent or aspect, both of which will be happy with a to.

In the examples provided by Andrew Tobilko, this would not hold. of is the only valid choice in those examples.

Generally, to can be used to relate qualities to objects. For example, it's natural and correct to say there is no flavour to this food - especially if you are a visitor to Britain.