How do you find a simple subject in a sentence?
The word that describes what we are talking about is the subject.
"The man read to his son" is a sentence. We are talking about the man. Man is the subject. I hope this helps. One of the English gurus can give a much more elegant answer but this cuts to the heart of the matter.
DrBob222 is exactly right.
Often the complete subject and the simple subject are identical, but not always. In the sentence DrBob gave you, that is the case, but if the sentence read like this, it'd be different.
The tall, gray-haired man read Wind in the Willows to his son for a class assignment.
In that sentence, "the tall, gray-haired man" serves as the complete subject -- that is the simple subject and its modifiers. The word "man," however, is the simple subject.
http://www.arts.uottawa.ca/writcent/hypergrammar/subjpred.html
http://library.thinkquest.org/2947/subjectsandpredicates.html
And here's a quiz to test yourself:
http://grammar.ccc.commnet.edu/grammar/quizzes/subjector.htm
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