When "Just because," is not a satisfactory answer.

Why can't a prepostional phrase be a subject complement? A question I have been trying to answer for months.
This is from A Student's Introduction to English Grammar, which is an abridged version of The Cambridge Grammar of the English Language. I am paraphrasing as the language is quite technical.

Prepositional phrases cannot be subject complements (a predicate adjective or predicate nominative). It does not matter if they are adjectival or adverbial. A complement must be licensed by the verb, meaning the complement can only occur with a certain type of verb that allows it. For a subject complement, this type of verb is a linking verb.

Here are some examples:

She is pretty. You cannot substitute a transitive or intransitive verb here: She ran pretty (Vi), She enjoyed (Vt) pretty. Neither work here. Pretty is a complement in this sentence, because you must have a linking verb in order for the sentence to be grammatical.

The book is about birds. You can substitute an intransitive: The book teaches about birds.

Jesus was about His Father's business. Jesus was learning about His Father's business.

She is from Nebraska. She comes from Nebraska. She walks from Nebraska.

She is in the woods. She sits in the woods. She walks in the woods.

She is in a hurry. She came in a hurry. She left in a hurry.

A word or phrase is only a subject complement if it requires a linking verb. If you can substitute any other type of verb, it loses its status as a complement.

Can weigh be a linking verb?

Here's the sentence: The roast weighs four pounds.

Is the word weigh a linking verb?

First, let's consider the other option:

Weighs is an intransitive verb and four pounds is an adverb telling how much to roast weighs.

When a verb is modified by an adverb, it describes the action being done by the subject. Since the roast is not doing any action,  four pounds cannot be adverbial. If there were action here, then the verb would be transitive and not intransitive since it is followed by a noun. That is, four pounds would become the direct object. If this is confusing, consider this sentence: The butcher weighs the roast. Here, weighs is a transitive verb and roast is the direct object.

So, yes, in this sentence, weighs is best classified as a linking verb. Four pounds is an attribute of the roast. It is a four pound roast.

Relative Clauses

She looked as if she needed a rest.

I have an English grammar book that cites as if she needed a rest as an example of a relative adverb clause. Here is the explanation:

As if she needed a rest tells how she looked.

What's wrong with that?

 Looked is a linking verb. To use a linking verb is to rename or describe the subject, adjectives describe nouns, therefore as if she needed a rest cannot be adverbial. If you are going to allow it to be adverbial, then in a sentence like I felt sick, sick is adverbial, because it tells how I felt.

Wait a minute, I was taught that to test for a linking verb, you can replace the verb with "to be" or "=" and if the meaning does not change, then it is a linking verb.

She = as if she needed a rest
That really doesn't work here.

The reason it does not work is because even though the clause is adjectival, it is not functioning as a predicate adjective (it is not a subject complement). If you want to know why, go here: http://livefullyawake.blogspot.com/2011/11/when-just-because-is-not-satisfactory.html

I will not name the text because overall, it is a reliable source and I do not wish to discredit it. But this demonstrates how engrained it is for us to classify words by what they tell us rather than by what they modify. 

But, couldn't as if she needed a rest be adverbial in this sentence?

Only if she looked with her eyes.

An Ah-Ha Moment

Here is the sentence:

I hid the gift in the closet.

When I was diagramming this sentence, I was baffled when I put "in the closet" under "hid" because it could be misinterpreted as: I hid in the closet. Therefore, I thought "in the closet" modified gifts instead. But I was wrong. I am documenting my thought process for the sake of fellow students of English grammar:

Here is what I considered:

There is an understood infinitive: I hid the gift (to be) in the closet.
There is an understood relative pronoun: I hid the gift (that is) in the closet.

My ah-ha moment came when I realized that "in the closet" could only mean "I hid in the closet" if "hid" was intransitive. Since it is a transitive verb, if the PP is understood as adverbial, it describes the action that is being done to the object.

For those of you scratching your head, here is the difference between transitive and intransitive verbs:
A transitive verb transfers the action of the verb from the subject to an object.
An intransitive verb does not transfer action from the subject to an object.

I hid -- intransitive, no direct object, an adverb would modify action done by the subject.
I hid the gift -- transitive, gift is the direct object, an adverb would modify action being done by the subject to the object.

Because the verb in the sentence is transitive, then "in the closet" does not tell where I hid, but where I did the hiding. I was in the closet when I hid the gift. That is, it describes not where I hid but where I hid the gift. That is not to say that "in the closet" could not modify gift, but you would need put it into a relative clause for the sake of clarity: I hid the gift that is in the closet.

So, what about this sentence:

I painted the chair in the corner.

Where I normally take a descriptive approach to grammar, I am going to be prescriptive here. If you want to be concise, you must write: I painted the chair that is in the corner.

The End Is Here

"The end is here."

could just as easily read...

"Here is the end."

One of the criteria of a predicate noun is that you can replace the subject with the predicate noun and it still makes sense. But, what are the other criteria? Here cannot take an article, like most nouns, so there is one strike against it already.

In the sentence“here is the end” one could argue that “here” is an expletive, not the subject, (like “there is the end”—you would diagram it with "there" above “the end | is”). The focus, when “be” is followed by an adverb, like now, here or soon is the existence of the end, much like I am using it here: It is inevitable that the end will be, it’s just a matter of when it will be. Here, I will concede that "is" could be an intransitive verb modified by an adverb.

However, this is not the same focus in a sentence like: The girl is in the kitchen. The question being answered is not whether or not she exists.

Here is what I woke up thinking about:

I painted the chair in the corner – If “in the corner” is adverbial, then it means that I sat in the corner and painted the chair. If it is adjectival, then it means that I painted the chair that is in the corner.

In the relative clause – that is in the corner – there is no question in my mind about what “in the corner” modifies, it is telling which chair by telling where the chair is. If you diagrammed it, you would make a dotted line from “that” to “chair”. If you replaced the pronoun with its antecedent, you would have: The chair is in the corner. (I have a math brain—it would be nice if I could plug in numbers to prove my point).

Like math, in grammar, it does not matter if you have 2 apples or 2 oranges it's still 2. Semantics do not determine grammatical function—meaning it is not fair to say “She is in the kitchen” really means “she is doing something in the kitchen” therefore “in the kitchen” is adverbial.

Think about common mistakes people make with linking verbs: I felt badly about the situation. They want to use the adverbial form of “bad” because it tells how they felt. (I am thinking of the time Donald Trump scathed Cyndi Lauper on national television for saying “I feel bad”. He argued that it should be “badly” and he is highly educated.)

{I am leaving the rest of my post here because I am blogging to document my learning process. But since I do not want to lead anyone astray with my former ignorance, here is a link to more accurate information: http://livefullyawake.blogspot.com/2011/11/when-just-because-is-not-satisfactory.html}

Why do we ask “How are you?” but have to answer with “what are you?”

Why would we let words that describe how I feel, look or seem be adjectives, but not words that tell where or when something IS? And why when there is a choice, why do we choose an adjective every time? Or if there is an adjectival equivalent, we can substitute it?

South Dakota and Montana are beside each other – They are adjacent or adjacently?

New York is in the north – New York is northern.