In short, in the context 1+1=2, and functions as a preposition
Hmmm...
Although your
intention here may be laudable (i.e. to provide a rational explanation for the rejection of unacceptable mathematical statements such as
*
One and one are two.
as opposed to acceptable, but formally less apparently defensible
One and one is two.)
, to attempt to label 'and' a preposition is, I'm afraid, not the right way to go about it!
As easy as it would be simply to point out that no respectable dictionary on earth would ever list it as one, I would rather persuade you with the reasoning process that underlies the way grammarians reckon its syntactic status.
Coordinating conjunctions, although primarily clause-connectors, can often function as simple word/phrase-connectors, e.g. 'between
Dover and
Calais' - not an abridged proposition of any kind, but a simple compound of two nouns. (The only preposition here, needless to say, is 'between'.)
While it is, of course, true that such a compound, when standing as the subject of a verb, will typically govern that verb in the plural, there are cases where, on the basis of unity of sense, the verb tends rather to be singular, as in
Bacon and eggs is a great way to start the day.
, as opposed to structurally unexceptionable, but semantically strange
!
Bacon and eggs are a great way to start the day.
, since, in speaking of 'bacon and eggs', we are referring to a single meal, a sense-wise indivisible commodity, rather than making an assertion relating to either 'eggs' or 'bacon' as individual items.
But this expressive facility does
not make the 'and' of the earlier sentence suddenly a preposition! It remains a conjunction, for the simple reason that
a preposition governs its noun in the objective case, while a conjunction simply connects, but does not govern.
We can demonstrate the difference by looking at the effects of a true preposition, e.g. the (phrasal) preposition 'together with'.
If we were to replace selected nouns in
[1]
The bus-driver, together with the tour guides, waited outside the church.
with pronouns, we would get
[2]
He, together with them, waited outside the church.
'He', as sentence-subject, is nominative, while 'them', as object of the preposition, is obligatorily objective-case.
If, on the other hand, we were to replace the preposition of [1] with the conjunction 'and', producing a semantically identical sentence,
[3]
The bus-driver, and the tour guides, waited outside the church.
and performed the same substitution exercise, we would get
[4]
He, and they, waited outside the church.
The morphology of the boldfaced pronouns in [2] and [4] is fixed entirely, and explicable only, according to the grammatical function of the preceding words. Moreover, there is no argument based on semantic intent that could ever make either
[5]
*He, together with they, waited outside the church.
or
[6]
*He, and them, waited outside the church.
grammatically acceptable.
Conclusion: whether serving as a clause-connector or as a phrase-connector, 'and' is only ever a conjunction.
QED
N.B. It will not escape the observant that essentially the same argument could be used to prove that 'plus' in standard, formal use is a preposition and not a conjunction, since we would say, e.g.
He, plus them,...
and not
*
He, plus they,...
