Identify parts of speech

Question

Three statements:

1. It weighs about a pound.

about = adjective or preposition??

'about' here adds information to the noun 'pound'. Therefore I believe it should be an adjective. But in regular sentences, 'about' functions either as a preposition or an adverb. So, what is it here?

2. He was only a yard off me.

I believe 'only' is an adjective because it adds information to the noun 'yard'. Am I thinking right?

'off' is a preposition in my opinion as it relates 'me' to the rest of the sentence.

3. I will watch while you sleep.

What is 'while' here?

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Additional & useful discussion on the first sentence: English Language & Usage

Answer 1

It weighs about a pound. 

After considering the discussion of this phrasing on English Language & Usage, I've changed my mind.  The "about" in "about a pound" is an adverb. 

Ordinarily, "a" or "an" is the indefinite article.  However, in this case, it is an ordinary determiner.  It's a synonym for "one". 

In this sentence, the phrase "about a pound" or "about one pound" is a noun phrase.  The word "about" modifies the word "a" or "one".  This modified determiner in turn modifies the noun "pound". 

When I first approached this sentence, I simply assumed that "a" was the indefinite article and that, as such, it wasn't an available to take an adverb as a modifier.  Under that assumption, it's easy enough to show that "about" does not modify the verb, the pronoun, or the noun.  Replacing "a" with a more explicit number does leave us with something that an adverb can modify.  If the grammar of "about five pounds", "about twenty pounds" and "about a hundred pounds" is the same as the grammar of "about a pound", then it makes sense to consider this instance of "about" to be an adverb. 

 
 

He was only a yard off me. 

In this sentence, the word "only" is an adverb.  It modifies the verb "was".  English allows fairly flexible placement of those adverbs which modifiy verbs:  "He only was a yard off me", "he was only a yard off me" and "he was a yard off me only" display this flexibility. 
 

The word "off" is a preposition.  The object of this preposition is the pronoun "me".  This prepositional phrase modifies the noun phrase "a yard". 

 
 

I will watch while you sleep. 

In this sentence, "you sleep" is a complete clause.  The subject is "you".  The verb is "sleep", which employs the active voice, present tense, indefinite aspect and indicative mode.  It's not the only clause in the sentence.  There is also a clause with "I" as the subject and "will watch" as the active voice, future tense, indefinite aspect verb. 

The word which connects these two clauses is "while". 

The phrase "while you sleep" modifies the verb of the matrix clause, making "you sleep" a subordinate clause.  A word which connects a subordinate clause to a matrix clause is a subordinating conjunction.

Answer 2

The parts of speech of "about" can be adverb, when it is used to express the meaning as "approximately" or " a little more or less than a number, amount....etc. Or to represent the number which is not gradable.

eg:

1. about six feet tall
2. about two months ago
3. about 4 O' clock ( about conveys here as "at" )
4. The car was exactly what I was looking for, and the price was about right. So I bought it. In this sentence "the price was about right" conveys that "the price was approximately correct or approximately what I wanted to pay
5. We were about ready to leave ( about = almost )
6. I have had about enough of your complaining. ( about = I do not want any more )
7. Well, I think that's about it for now. Shall we tidy up ready to start again tomorrow. ( about = We have almost finished what we are doing for the present)

Now the question is, how "about" becomes adverb in the sentence "It weighs about a pound"? From above explanation, "about a pound" means, "approximately a pound"

Here "pound" is noun. And "a pound" means " one pound". The number ( one or "a") is adjective.

As per the definition of adverb : "An Adverb is a word used to add something to the meaning of a verb, an adjective, or another adverb"

so, "about" is adverb. Because it adds some meaning ( approximately ) to the quantity ( one / "a" ) of the unit ( pound ). Therefore the parts of speech of "about" in the sentence "It weighs about a pound", is adverb.

Similar example : He is about six feet tall.

In this sentence, "feet" is noun ( unit of measure, a noun). "six" is number, an adjective. "about" (= approximately ) adds meaning to the number ( six ), so "about" is adverb.

When we apply same principle to all examples as shown below, where "about" can be classified as "adverb"

1. He is about six feet tall
2. I saw him about two months ago
3. The time is about 4 O' clock ( about conveys here as "at" )
4. The car was exactly what I was looking for, and the price was about right. So I bought it.
5. We were about ready to leave on that day.
6. I have had about enough of your complaining.
7. Well, I think that's about it for now. Shall we tidy up ready to start again tomorrow.

-courtesy

1. Cambridge International Dictionary of English
2. Wren and Martin, High School English Grammar and composition.

Answer 3

Without the word "about", the sentence is simple:

It weighs a pound.

That's a pronoun, a verb, and a noun phrase acting as a subject, a transitive verb, and a direct object.

Even with the word "about", much of the grammar of the sentence remains the same:

It weighs about a pound.

"It" remains the subject, "weighs" remains a transitive verb, and the phrase "about a pound" might still be the direct object.

I think we can immediate discount the possibilities that this "about" is a noun, verb, conjunction or interjection.  We only need to examine the preposition, the adjective and the adverb as reasonable possibilities.  
 

The word "about" is often a preposition, and "about a pound" could be a prepositional phrase.  If it is, we should be able to find other prepositional phrases which fit this sentence pattern.

We learned early American history.
We learned about the pilgrims.

I cleaned the floor.
I cleaned under the bed.

There is a clear general pattern here, and there are at least two ways to explain it.  One obvious explanation is that prepositional phrases can be direct objects.  Another explanation is that prepositional phrases can modify the intransitive use of a normally transitive verb.  In either case, the pattern Subject / Verb / Prepositional Phrase make sense.

This is reasonable evidence for the "about" in question being a preposition.  
 

If it is an ordinary adjective, it would appear after the definite article: "an about pound".  If it modifies "a pound" at all, it must be a predeterminer.  Unfortunately, I don't see how to distinguish between this possibility and the possibility that it is a preposition.

Instead, I'll rely on Oxford Dictionaries.  That source lists only preposition and adverb as possible grammatical categories.  In contrast, its listing for all does include the category predeterminer.

Either Oxford Dictionaries is mistaken, or the word "about" is not a predeterminer.  
 

If it is an adverb, then it must modify the verb.  There is no other word in this sentence that an adverb could modify.  If so, we should be free to change its placement:

It about weighs a pound. (x)
It weighs about a pound.
It weighs a pound about. (x)

In comparison:

It only weighs a pound.
It weighs only a pound.
It weighs a pound only.

This is sufficient reason to claim that the "about" in question is not an adverb. 

 
 
In the sentence "It weighs about a pound", "about" is a preposition and "a pound" is that preposition's object.

Answer 4

about in 'It weighs about a pound' indicates that the weight of 'it' is close to a pound. it shows nearness. Hence it is a preposition.