Both of the sentences you presented are grammatically flawed:
Let’s analyze the first one:
“The old man sat on the sofa, his face being serious.”
The modifying phrase “his face being serious” is grammatically dysfunctional because of the presence of the verb “being.” It should be dropped to make that phrase work properly, as follows:
“The old man sat on the sofa,
his face serious.”
(The sentence works properly without the verb “being” in the modifying phrase “his face
being serious.)
The second sentence you presented is likewise grammatically dysfunctional because of the presence of the verb “is.” As in the case of the verb “being” in the first sentence, it should also be dropped for that phrase to work properly, as follows:
“The old man sat on the sofa,
his face serious.”
(The sentence works properly without the verb “is” in the modifying phrase “his face
is serious.)
The two modifying phrases above whose operative verbs were taken out are known as
absolute phrases or
nominative absolutes. Unlike the usual modifying phrase, an absolute phrase doesn’t directly modify a specific word in the main clause of the sentence. Instead, it typically modifies the entire main clause, adding information or providing context to it.
In particular, the absolute phrase “his face serious” in the two corrected sentences above belongs to the most common form of the absolute phrase. In that form, a modifying word or phrase is tacked on to a noun or pronoun without using a verb, preposition, or conjunction, as in these examples: “
His resolve weaker, the boxer gave up the fight.” “
Its headlamps dim, the car rammed the lamppost.” This form of the absolute phrase can even be a single word (either a past participle or a present participle), as in these examples: “
Miffed, the unwilling candidate snubbed his own proclamation.” “
Wavering, the troops finally raised the white flag.”
Notice that the four sentences above that are modified by absolute phrases are actually elliptical or streamlined forms of the following sentences that are modified by participial or adverbial phrases: “
The resolve of the boxer was weaker, so he gave up the fight.” “
Its headlamps were dim, so the car rammed the lamppost.” “
The unwilling candidate was miffed, so he snubbed his own proclamation.” “
With them wavering, the troops finally raised the white flag.” By using absolute phrases as modifiers, the four sentences are able to get rid of the weak verb forms “was” and “were” and become simpler and more concise sentences.
I trust that this explanation adequately answers your question about the two grammatically flawed sentences you presented. For a more comprehensive discussion of the absolute phrases, I suggest you check out my book
Give Your English the Winning Edge (Manila Times Publishing, 486 pages). It devotes three chapters to explain the intricacies of this admittedly baffling aspect of English grammar.
