This is an interesting question, particularly because of this dichotomy:
- This is a type of apple. (not apples)
- These are two types of apples. (not apple)
I think that the construction of the form "two types of apple" sounds more than stilted; it just plain sounds awkward, and I would be surprised that it sounds familiar and normal to anyone (at least speaking for US English).
The idea that a plural form would be used for a class is actually not strange at all in English. To express the fact that I like things belonging to the "apple" class, I would say:
I would not be able to use the singular to express this:
- *I like apple.
- *I like an apple.
- *I like the apple.
Saying "I like apples" doesn't even imply that I am talking about multiple apples; one could say this, for example:
- I like apples, although I've only ever had one in my life.
So, saying "I like all sorts of apples" seems to jibe perfectly with the rest of English grammar.
This means that the strange case is actually this one:
- This is a type of apple. (not apples)
Saying "this is a type of apples" is definitely not natural or familiar. It seems that, in phrases like "type(s) of X" ("kind(s) of X", etc.), there is generally number concord between the type-word and the class itself. Why that is, I don't know.
As Sam said in his comment, the crux is what precedes the "this/these include(s)", not what follows. As a matter of fact, "This includes administrators and normal users" can be just fine, depending on the context.
That said, these can still be a little tricky. For example:
Several different users can change their own passwords; these include administrators, and normal users.
The "these include" refers to "several different users." But:
No users are allowed to change their own passwords; this includes administrators, and normal users.
In this case, the "this includes" is singular, because it refers to the single principle that no user is allowed to change a password.
Corruption was uncovered at many levels across the organization; these include both administrators, and normal users.
Here, "these include" refers back to the many levels where corruption was found.
However, instead of trying to figure out the correct usage, a better option would be to use the more all-encompassing including. In fact, this change seems to improve the sentences:
Several different users can change their own passwords, including administrators and normal users.
No users are allowed to change their own passwords; including both administrators and normal users.
Corruption was uncovered at many levels across the organization, including administrators and normal users.
Get rid of this this, and this problem goes away.
A footnote (to underscore how tricky the this/these choice can be): I could have said, "In fact, these changes seem to improve the sentences..."
Which is correct? If I am referring to the single hint of replacing the phrase "this includes" with the word including, then "this change seems to improve" is appropriate. But, if I'm referring to the three individual improvements found in each example, then "these changes" would be better.
Best Answer
Kind(s) of (like sort of, breed of, manner of, variety of, and so on) is slippery. It is usually interpreted as referring to the concept of something: uncountable, and taking a singular object. On the other hand, it can also refer to a set of them— or to a set of concepts— and the object would thus be plural.
I would expect to see
and not so much
but I cannot ascribe this to any particular grammatical rule, only familiarity (in American English) and style. After all, in informal conversation we could ask
and expect a plural answer, as if you had asked
which would sound stilted unless you were trying to make small talk with a librarian or bookbinder about classifications of books.
We also find similar constructions in more formal or archaic English, e.g.
As an aside, I love TheOatmeal, but do not demand precise use of language from web comics.