Adverb or preposition?

Question

What does the word 'in' function as in the sentence - "Please, come in"?

It seems like an adverb to me. As, 'here' is an adverb in - "Come here" and 'in' functions quite similarly in the above sentence.

Answer 1

Please, come in.

In this sentence come in. is a phrasal verb, where in is a particle. In Modern Grammar this particle is classified as a preposition.

For further discussion please refer to below -

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Let's consider the following two sentences -

1. He jumped off the wall.

2. He took off the price tag.

In sentence #1 the Verb Phrase - jumped off the wall - contains only one Complement. The complement - off the wall - is a Prepositional Phrase, where off is the head of the phrase - a Preposition. The noun phrase - the wall - is the complement of the preposition - off - in the Prepositional Phrase structure.

Sentence #2, though it looks similar to sentence #1, is structurally different. Here the Verb Phrase - took off the price tag - contains not the one complement, but two. The complements are - off (a Prepositional Phrase) and the price tag (a Noun Phrase). The Prepositional Phrase - off - has no complement of its own, and hence this Prepositional Phrase contains only a preposition with no complement.

The Prepositional Phrase - off - can either precede the direct object (here in sentence #2 it's a noun phrase - the price tag) or follow it, as in the following -

3. He took the price tag off.

This Prepositional Phrase - off is called a Particle. (CGEL says complements which precedes a direct object in this way is a particle. But I have seen in many places, like in BBC website, where it is told that such complements are Particles, regardless of their position.)

Whether you call it a Particle or not, off is actually a Preposition in sentence #2.
(Whatever I wrote here is based on Modern Grammar. Traditional Grammar differs in explanation.)

NB -

In many cases like this one there are often conflicts between traditional grammar and modern grammar. Generally what I do is I accept both of them, and don't allow the idea that one is better or the other is not up to the mark or something like that as long as they explain everything and don't have any restrictions or too much exceptions and don't complicate things. But in this case I prefer the modern explanations over the traditional one.

WHY?

For such verb + particle, in traditional grammar there are three categorizations -
i) Phrasal Verb
ii) Prepositional verb
iii) Prepositional Phrasal verb

But their definitions are not that clear. In phrasal verb it says it always take an adverb after the verb. But a phrasal verb can have an object or it can be without object.
On the other hand the traditional definition of a preposition says that it should have a noun phrase as complement. And when there is no complement it is called an adverb. Now that contradicts.
Traditional grammar says take off is a phrasal verb.

a) Why don't you take off your coat?
b) He came in.

In sentence #a the phrasal verb has an object, in sentence #b the phrasal verb don't have any object. But according to the definition of phrasal verb, both in and off in sentence #a and #b are adverbs. But the traditional definition of adverb and preposition contradicts.

In traditional definition of Prepositional verb, it says the verb and the particles are inseparable and the particles are actually a preposition.
It says refer to is a prepositional verb, and so to is a preposition and is not inseparable.
But as to is a preposition it has to take an object according to the traditional definition of preposition. So if it has the object how it's different from the phrasal verb which takes an object?
Traditional grammar says it's inseparable. But in practice we often separate them. For example -

c) These are two of a very large number of similar actions brought by various banks against various local authorities, in respect of which a lead action order has been made to which I refer below.

REFERENCES -

1. Cambridge Grammar of English Language (CGEL) page 54 and 58

2. BBC website

3. Traditional discussion of Phrasal Verb

Answer 2

Yes it indeed is an adverb. As seen in this site:

Please come in

is listed as under the heading of adverb.

Answer 3

Please, come in.

The OP is right. The word "in" in the sentence is an adverb, not a preposition, that means inside or indoors.

You usually use a noun, noun phrase, pronoun, or a gerund after the "in" when it's used as a preposition, whereas there are usually no such words after the "in" when it's used as an adverb.

Answer 4

If a preposition after a verb is used without noun it is treated as an adverb in traditional grammar. As such particles are part of a compound verb "compound element" or "compound particle" would be a better term.