Is there any real difference between "to" as a preposition and "to" as an adverb?

I'm really in doubt. On the free dictionary I read this concept of "to" as a preposition: "1. (used for expressing motion or direction toward a place, person, or thing approached and reached): Come to the house." Right below I read this concept of "to" as an adverb: "18. toward a point, person, place, or thing."

For me, there's no difference. Is there any subtle thing I cannot see that makes the difference? Thanks.

source: http://www.thefreedictionary.com/to

Sadly, the most influential book on English Grammar does not accept the unqualified statement Prepositions are followed by a noun phrase or pronoun. It accepts the idea of the subset of 'intransitive prepositions' which are obviously not followed by a noun phrase or pronoun, citing the similarity of structures such as:

He had never been abroad before the war.

He had never been abroad before.

However, many grammarians do maintain that this analysis is imperfect.

If we accept that prepositions need a following noun group, before is readily classed as a preposition in the first sentence here and as an adverb in the second (compare he went abroad later).

However, this isn't a complete explanation either. In

The ship hove to.

The 'to' is obviously not a preposition - but it cannot be dropped from the sentence without leaving an ungrammatical residue. This is different from the above example using an adverb:

He had never been abroad before.

=> He had never been abroad. (OK)

In such cases, an analysis I subscribe to is that the 'heave to' construction cannot be separated into different parts - it's a multi-word lexeme (cf 'ship of the desert' for camel). If the 'to' in such a structure needs an individual name, the one it usually ends up with is 'particle', or, to show its affiliations, 'adverbial particle'.

I'd argue that 'bring to' and 'fall to' in Shoe's examples are multi-word lexemes.

A drop of whiskey brought him to. *A drop of whiskey brought him.

Fall to! DM:Fall!

Examples of to as a true adverb don't seem very common - but he may have found one in 'She pushed the door to.' But perhaps this is better analysed yet another way, as a directional / locative particle: 'I think she pushed the bolt to/home. Is it home?'

Apparently, a nautical expression uses 'to' as meaning 'into the wind', so this is (arguably! - how cohesive are turn and to here?) an adverbial usage:

We turned to and flew as much canvas as we could manage... (Google)

Prepositions are followed by a noun phrase or pronoun. So in these sentences, to functions as a preposition:

She's gone to the bank.

He ran to the gate.

Come to me.

However, to can also function as an adverb (the bucket into which all words that are not clearly another part of speech are typically dumped). In the following sentences to is not followed by a pronoun or noun phrase and can be considered an adverb:

She pushed the door to.

A drop of whiskey brought him to.

Fall to!

Note: The following sentence also ends with to:

Which shop did you go to?

In such cases, however, to can be regarded as a preposition separated from its pronoun/noun phrase:

To which shop did you go?