when to use singular and plural

I have a few questions about this, but perhaps I'll just start with one. Can you say "The parents of an infant have equal rights as guardian of the infant's property"? Note that it's guardian and not guardians. Thanks.

Your nouns need to match. Since "parents" is plural, then "guardian" should also be plural; thus, "guardians" would be the correct choice.

If we were to consider "parents" as one entity would it be all right? How about "We constantly refer to the meaning and intent of the constitutional provisions and its impact." I find changing the its to their rather awkward. What do you think? Thanks.

No, I'd change "its" to "their." When you use the plural, you have to write accordingly.

Yes, if you're used to speaking and writing English, switching every element in a sentence to the plural form simply because of a plural noun wouldn't seem so awkward. Isn't one supposed to do that in French too?

aah, ok. Well, I bought a book published by Oxford and it comes with a tape. Anyway, in one extract the speaker says "payments are not the problem." How is this different to the sentences mentioned above? Shouldn't it be "problems"? One more example would be an entire chapter and a few listening exercises dedicated to "Types of Negotiation" and not "negotiations". ??
In 'Practical English' by Madeline Semmelmeyer (rather old grammar book) an example of two subjcts forming a unit is given - 1) Bacon and eggs is a popular combination. Do you know of any other exception to the plural rule?
BTW, you have all been (a?) great help.

"Payments are not the problem."

This does necessitate the plural "problems" because "payments" could form one category. For example, if I were a business owner and clients weren't making their payments on time, this could be a big problem for me. Thus, payments are a problem....i.e. I'm talking about payments as a whole rather than individually.

*TYPO......

"Payments are not the problem."

"This DOES NOT necessitate the plural problems..."

Sorry, I left out the "not."

TQ so much. Have you got any explanation for "The negotiation" bit?

Again, think of "negotiation" as a whole, a category in itself. They're not talking about different types of negotiations, but negotiation in general.