The subject of a sentence is often who or what performs the action of the verb.
Lucinda sang a solo.
Voices filled the auditorium.
It can also be on whom or what an action is performed.
The singers were assigned roles.
The music was chosen by the choral director.
Sometimes the subject is merely in the condition indicated by the verb.
Oscar is a tenor.
The curtain is open.
The subject is not always found at the beginning of the sentence.
After the concert the singers went to a party.
It can even be after the verb.
In the concert program are photographs of the soloists.
The grammatical subject is not always what the sentence is about.
I found the singing beautiful. (The subject is I, but the sentence is about the singing.)
Two or more subjects joined by and form a compound subject.
Tenors and sopranos rehearse on Wednesdays.
Oscar, Nate, and Luis are tenors.