I think I confused more than helped in my last post about "coming with..." I am going to elaborate a bit on the different uses that words that are commonly prepositions might have in writing:
1. First of all, a word is seldom a certain part of speech in isolation. Words are called parts of speech because they are used in a certain way in speech (and writing). Thus, it is often incorrect to say that, for instance, a dog is a noun. You can be dog tired. You can dog somebody to pay you. A part of speech is a part of speech when it is used--not in isolation.
2. Thus, the preposition as other parts of speech problem. We have students memorize lists of prepositions (though we prefer to have them use them in Check Sentences, again, because that is how "parts of speech" are used)--but we have to remember that those prepositions are only prepositions when they are used as prepositions--how is that for confusing? Remember, a preposition must have an object following it in order to be considered as being used as a preposition.
3. Examples!!! I will list prepositions below to show how they may be used as prepositions or how they may be used as other parts of speech--again, in context.
i. I am coming over. (Adverb--tells where you are coming....)
ii. Jump over the water. (Preposition--begins the prepositional phrase (PP for short): over the water...)
i. He fell down. (Adverb--tells where he fell..)
ii. We rode down the hill. (Preposition--begins the PP down the hill...)
i. Before we go to class, let's check our backpacks. (Subordinator--before is used as a subordinator beginning the subordinate clause before we go to class--a subordinate clause is a clause (subject/verb) that begins with a subordinator and is not a real sentence by itself.)
ii. I heard that story before. (Adverb--tells when you heard that story...)
iii. He has to go before the leaders. (Preposition--begins the PP before the leaders...)
Hope this helps! Feel free to write in questions--if I don't know the answer, I will look the question up in my 600 page reference! :)