manner or simultaneously?

Question

There is such a statement in the book below. it mentioned that it is simultaneous. but can't we also think of it as "manner"? I understand this sentence in this way at the same time. Because it answers the question ''how'' . What is the difference?

Source : A University Course in English Grammar an action (main clause) and a mental process (-ing clause) occurring simultaneously:

They drove on, wondering how long their petrol would last. (additive = and wondered)

They drove on and wondered how long their petrol would last.

And the statement in another book is this: Source : Compact Advanced Student's Book description of the subject of the main clause; the participle clause normally comes second:

The volunteers arrived at the clinic feeling a little nervous.

This sentence also answers the question "how". Also this sentence is a ''manner'' sentence.

Answer 1

The difference is the verb. In the first you have "wondering". This is an action that they are taking. It tells you what they were doing.

But the second is a verb that that describes an internal state. If you "feel" something, this verb is not something that you do, but something that you are.

As a main clause, the distinction remains:

I feel nervous (feel is a linking verb, nervous is a complement)

I wonder {what it is} (Not a linking verb)