First, the subject and predicate terms of a standard-form categorical proposition always designate classes, but those terms may be complicated expressions rather than single words. The important point about this is to recognize that whether a class of things or actions is easily named with a single word or requires a paragraph-like specification, it is still one kind of thing or action that we happen to be talking about. Closely related to the notion that the subject and predicate terms of categorical claims designate classes is the requirement that classes are always designated by nouns or noun phrases. What this means is that categories, even when they are actions, are treated as groupings of somehow homogeneous things. English and many other languages allow us to create sentences that look similar on the surface, but are really significantly different.
Dogs are fun.
The easiest way to do this is to recast the second sentence. Dogs are things that are fun. Depending upon whether the ambiguous original was intended as a true universal affirmative or something a liitle less absolute, there will be different translations of the claim into standard form. If one wanted to allow for even one exception, the sentence above would be translated into a particular affirmative claim, the kind of categorical claim that is used to allow for some possible exceptions, while still conveying a general idea. Some dogs are things that are fun.