Identify subject of a sentence

The subject is the noun the sentence is built around.
Find the verb first, then ask "who or what" is doing that verb.
Whatever answers that question is the subject.

When a sentence starts with a describing phrase, that phrase describes the subject that comes right after it.
In "Upset by the litter, Jane started a cleanup," the phrase "Upset by the litter" describes Jane.
So the subject is Jane — she is the one who is upset.

In a phrase like "X, such as Y," the subject is X, not the example Y.
In "Vertical flutes, such as the recorder, are easy to play," the subject is "vertical flutes," and "the recorder" is only an example.
The verb "are" agrees with the flutes, not the recorder.

Watch out for extra words between the subject and the verb.
In "The box of old letters was heavy," the subject is "box," not "letters."
Strip away "of old letters" and you are left with "The box was heavy."

What is the subject of "The children in the park laughed loudly"?
The verb is "laughed." Ask: who or what laughed?
The children laughed, so "children" is the subject.

In "Excited about the trip, Maria packed her bag," who does the phrase "Excited about the trip" describe?
It describes the person who comes right after it, Maria.
So the subject is Maria — she is the one who is excited.

In "Citrus fruits, such as lemons, are rich in vitamin C," what is the subject?
The subject is "citrus fruits," and "lemons" is only an example.
The verb "are" matches the fruits, not the lemons.