Question

what is the subject of a sentence?

Answer 1

Usually the noun, or the pronoun, subject is what the sentense describes. For example,

Did you see my blue hat?
The blue hat being a subject here.

Answer 2

What @NeetziD said is correct; its the noun/pronoun being described in a sentence. For more, check http://www.chompchomp.com/terms/subject.htm

Answer 3

what ever usually stands out as the subject or the topic it is talking about.

Answer 4

This may help: http://is.gd/0hBKLg