Mark stood next to Tracy, feeling helplessly nervous.
^ This means that Mark was standing next to Tracy and he was feeling nervous. It cannot mean that Tracy was feeling nervous.
Mark stood next to Tracy, who was feeling helpless nervous.
^ This means that Mark was standing next to Tracy, and that Tracy was feeling nervous. It cannot mean that Mark was feeling nervous.
The general rule is that if you have an independent clause (words that can make a complete sentence on their own), followed by a comma, followed by an adjective (or phrase that acts like an adjective), without a noun or pronoun that acts as a subject, the adjective modifies the subject of the sentence.. This is a basic grammar construct you will see again and again. The adjective never modifies the object of the sentence or another noun in the sentence in these cases; context does not matter.
Mark stood next to Tracy, feeling helplessly nervous.
^ Here Mark
is the subject, next to Tracy
is an adverbial clause that modifies stood
, and the phrase feeling helplessly nervous
functions as an adjective. There is no noun or pronoun in feeling helplessly nervous
, so the grammar rule applies. Feeling
can only modify the subject, Mark
.
Mark stood next to Tracy, who was feeling helpless nervous.
^ Here the entire phrase next to Tracy, who was feeling helpless nervous
, functions as an adverb which modifies stood
. Within that phrase, Tracy, who was feeling helpless nervous
functions as a single noun phrase.
It's also worth noting that the adjectival phrase can occur at many places in the sentence:
Mark stood next to Tracy, feeling helplessly nervous.
Mark, feeling totally helpless, stood next to Tracy.
Feeling helplessly nervous, Mark stood next to Tracy.
In all of these three, feeling
modifies Mark
, the subject of the sentence. Adjective clause that consist of <verb> + ing
set off by a comma always refer to the subject of the clause they modify.