In my Oxford dictionary it says that the word plus amongst other roles can be either a preposition or a conjunction.

Examples of “plus” acting as a preposition:

He was awarded the full amount plus interest.

All apartments have a small kitchen plus private bathroom.

The temperature is frequently plus 35 degrees at midday.

Examples of “plus” acting as a conjunction:

It’s packed full of medical advice, plus it keeps you informed about the latest research.

Now my question is that to me all the examples of “plus” acting as preposition seem confusing, because to me “plus” in all those examples is acting as a mere conjunction, I do not get how “plus” is acting as preposition in those examples!

My Canadian Oxford Paperback Dictionary lists 'plus' as a preposition but not as a conjuction.

It also says 'The use of plus as a conjunction is considered incorrect by some. It is to be avoided in formal writing.' I heartily agree.

With regard to your question, I suppose it depends on your definition of a preposition.

To me, the main point is that the meaning of each of your examples is clear.

Best wishes, Clive
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A conjunction combines to groups of words that can stand alone as sentences. (There are a few different tweeks, but you can get the jist of it) The reason we use conjunctions is so we can better package one full idea. The word plus in, "It’s packed full of medical advice, plus it keeps you informed about the latest research." is used as a conjunction because it combines two groups of words together that can stand alone as seperate sentences. Plus in, "He was awarded the full amount plus interest." is used as preposition. A preposition is a word that describes the relationship of a noun to another noun (object of the preposition). Can you see the relationship between "amount" and "interest"? In conclusion, while concentrating on both of these forms of the same word the realization will arise that they are both functioning very differently.

In many of the uses of plus as a preposition, you can substitute "with" or "also" for "plus". The example of plus as conjunction is a construction where one could substitute "and" for "plus", although for least one of the preposition examples "and" could also be substituted. However, the example of "plus" as conjunction shows it being used to make a compound sentence.

This is not how prepositions and conjunctions are usually defined. Conjunction: "Apples and oranges", "red or blue" -- where are the two sentences? Preposition: "It rains in the mountains" -- where is the other noun that is related to "mountains"?

I agree with the OP that in all examples "plus" seems to act as a preposition. With one exception: "plus 35 degrees". In math you would call that a unary plus. I don't know what it's called in grammar. "Plus" modifies the numeral 35 as an adverb would modify an adjective. Wiktionary lists that use under "adjective" which I think is inaccurate.

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