The idea of an equational sentence is fairly simple, it is a sentence with a subject and a use of the "to be verb." It is called an equational sentence because it closely resembles an arithmetic equation. Here is an example:

"Everything2 is everything I want in a website."

"Everything2" is the subject of the sentence, let us call it "X." "Everything I want in a website" defines what the subject exists as, let us call it "Y." The word "is" is like an equal sign. Let's see that example again in mathspeak:

X = Y.

It makes me wonder if you can consider a math equation in terms of subjects and predicates. If I say, "A + 4 = 24," can "A + 4" be considered the subject of the sentence with "24" being the predicate?