On my last question, I asked if "looking for sea shells" was a non-restrictive clause in the sentence, "I was at the lake[,] looking for sea shells, but instead, I found this dollar bill." I was told that it was restrictive. I've asked a question quite a while ago, and it read: is the phrase, "looking for water," non-restrictive in the sentence, "I'm at the desert[,] looking for water"? I was told that it was a non-restrictive clause and therefore needs a comma before it. So why is "looking for water" non-restrictive but "looking for sea shells" isn't? Can someone clear this up for me?
Let's look at a simpler sentence first:
I am happy.
We can split this clause into a subject and a predicate. The subject is "I", and the predicate is "am happy". We can split that predicate into a verb and its argument: "am" and "happy", respectively.
It is true that "happy" modifies the subject. However, it doesn't directly modify that subject. Instead, it acts as the argument of the verb, specifically as a subject complement. There is no point in asking whether it is a restrictive or non-restrictive modifier because it isn't a direct modifier.
Things are different if the modifier belongs directly to the subject. The "happy" of "A happy man sat at the table" is a restrictive modifier.
Here are two more sentences:
I am in the desert.
I am looking for water.
These two sentences can be described in exactly the same way as the first sentence. We can say that "in the desert" and "looking for water" are subject complements.
A final two sentences:
I am in the desert and looking for water.
I am in the desert, looking for water.
One of these sentences joins a pair of subject complements with a conjunction. The other joins that same pair with nothing more than a comma.
The existence of the comma in the last sentence has nothing to do with non-restrictive modifiers. Instead, the comma is there because otherwise it is the desert that is looking for water.