The subjectThe person or thing that performs the action of a verb or that is described by a verb. E.g.: Andy has never been to Brunei. ("Andy") of the actionA general term expressing that somebody or something does something expressed by a participle is usually the nounA part of speech that names a person, place or thing. E.g.: The koala is a marsupial. ("koala", "marsupial") or pronounA part of speech that refers to or stands instead of a noun. E.g.: Who do you think I am? ("you", "I") preceding it or, if there is nothing in this position, it is the subject of the following main verbA verb that can express an action or state in itself. E.g.: John likes squash a lot. ("like").

I saw him crossing the street.
Who is the girl sitting next to you in class?
I was going home when the accident happened.
Having taken the wrong turn, he ended up in a dangerous suburb.

(The wrong turn is part of the non-finite clauseA group of related words containing a subject and a verb. and it is actually the objectA person or thing that is affected by the action of a verb or involved in the result of an action. of the participle having taken.)

The position of a participle in the sentence is crucial to understanding who or what its subject is:

Driven by desperate hunger, he ate raw fish. (the subject of driven is he)
He ate raw fish, driven by desperate hunger. (the subject of driven is raw fish)

The second sentence above is incorrect because the participle is misrelated, which means that it does not refer to the subject intended. Here is another example:

Going home, a bus almost ran over me.

The subject of infinitives
The subject of non-finite forms The subject of gerunds